w/CS: Assembling California*, John McPhee

July 2023

* Reading with CJS. Page numbers cited are from Annals of the Former World, a collection of his work of which Assembling California is only one part. The parts of the AC, as described below, are not well-separated in the text; they only appear in the table of contents.

The Gold Rush of the Nineteenth Century

While this is something I’m not keenly interested in — it caught my interest as a pre-teen and adolescent and perhaps I enjoyed a surfeit of it — it was well written, and, striking in that it revealed the immense scope of anthropogenic change caused by mining, and particularly hydraulic mining. Hydraulic mining was practical because there were a lot of ‘fossil’ river beds in which gold had accumulated for millennia, and which — with a high pressure jet of water — was there for the taking.

One interesting thing I learned is that there is a geological feature called a horse. I initially thought McPhee was making a joke — that he mean “horst,” which is an elevated block bounded my normal faults, and usually occurring along with grabbens, which are blocks that had dropped in elevation. But, in fact, a “horse” is a name for any block that is completely separated from surrounding material by mineral veins or faults of any kind. (The term is used in reference of Mussel Rock, in the Ocean View area of San Francisco.)

The piece also discussed the ways in which gold was concentrated, and re-concentrated, by tectonic processes in play as the terranes that now make up California collided with the west coast of proto-North America [Laurentia?].

Some other things I learned:

With the exception of volcanoes, mountains consist of whatever happened to be there when tectonics made them.
Sixteen miles west of Donner Summit, beside a bridge over the road to Yuba Gap, there is a roadcut showing “mostly andesite mudflow breccia with reworked stream gravel in it and glacial till on top.”
The Sierra Nevada batholith was created in successive pulses over 130 million years. There were three peaks at 250 Ma, 140 Ma, and 80 Ma. The last was most extensive.
For eleven miles(heading west on I80) after Donner Summit, the xenoliths increased in size from pebbles to bears, indicating that we are approaching the wall of the magma chamber. There is a vertical contact marking the wall of the batholith in a road cut.
Terranes coming via the ocean are said to “dock” or to be sutured to the continent.
There were three terrains that got sutured to North America. The first, called XXXX, arrived during the Mississippian period and thrust itself almost to Utah. The second was Sonomia. The third arrived in the Mesozoic, and smashed into Sonomia with “crumpling, mountain-building effects tat would prorate eastward through the whole of Sonoma, metamorphosing its sediments … and folding them at least twice.
Most of the world’s great batholiths are not quite true granite; they are more maffic, and thus technically speaking are granodiorite.
Prior to plate techtonics, geosynclines were used to explain a lot of the structure visible in the earth.
A lot of gold was mined from ‘fossil rivers:’ “50 million years before the present, they had come down from the east of a very high plateau to cross low country that is now California, and leave their sorted bed loads on a tropical coastal plain.”
I80 past Gold Run: “The roadside records the abrupt change. As if you were swinging off a riverbank and dropping into the water, you go out of the metavolcanic rock and into the auriferous gravels. We stopped, stood on the shoulder, and looked about a hundred feet up an escarpment that resembled an excavated roadcut but had not been excavated by highway engineers. It was capped by a mat of forest floor, raggedly overhanging. The forest, if you could call it that, was a narrow stand of ponderosas, above an understory of manzanita with round fleshy leaves and dark-red bark. The auriferous gravels were russet, and were full of cobbles the size of tomatoes- large stones of long transport by a most impressive river.”
Before hydraulic mining, the elevation of the Sacramento river was sea level; afterwards it was seven feet.

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Ophiolites and the Smartsville Block

Traveling along I80 from east to west:

Soon we were dropping toward two thousand feet, among deeply weathered walls of phyllite, in color cherry and claret–the preserved soils of the subtropics when the unrisen mountains were a coastal plain. Geologists call it lateritic soil, in homage to the Latin word for brick. All around the Sierra, between two and three thousand feet of altitude, is a band of red soil, its color deepened by rainfall that leaches out competing colors and intensifies the iron oxide. Not only phyllites but also mica schists, shales, tuffs, and sandstones in the roadcuts were red. When the road dipped far below the rooflike plane of the western Sierra Nevada, the dissected inclines around us had the appearance of red mountains covered with manzanita.

At Weimar, a little off the highway and close to the two-thousand-foot contour, was a narrow band of serpentine, the California state rock. Moores said, “Worldwide, there is an association between serpentine and gold-bearing quartz…
— John McPhee, Assembling California, p. 475

Sheeted dikes — as narrow as 10 cm and as wide as 80 cm – of diabase at Auburn*. New dikes intrude every 50-100 years. The sheets are vertical, and older sheets are pushed outward to one side or another. By looking closely at their edges, you could all that to see the spreading center that the accumulating rock had slowly moved away from. Layer after layer was glassy along its right hand edge. The bag might have a cool quickly there after touching solidified rock. The spreading center, therefore, have been to the left. After a new lamination of magma touched hard rock and turned marginally to glass, the rest of the lamination froze, more slowly, forming to find crystals. Some layers had class emergent on both sides. They had to split the week center of previous and still cooling layers.” [p 478]
* In Auburn ravine, a couple of hundred yards below the railroad overpass at exactly 1200 ft above sea level the interstate had been cut though charcoal gray rock. …about a mile from Auburn dam.
Three terranes: “The three terranes extended the continent by at least 400 miles. The third one, suturing here [at Auburn], had doubled the width of what is now California. The sheet of diabase that we found in Auburn – shattered so grossly in the collision – was a part of the ocean crust is the leading edge of the third terrane.” [479-480]
Formation of the Mother Lode. “If you look at a map of the Mother Lode and lode-gold belts related to it — a narrow band, north-south, lying under Grass Valley, Forest Hill, Placerville, Plymouth, Mokelumne Hill, Angels Camp, Carson Hill — you are, for practical purposes, looking at a map of the Smartville suture. As a geologically immediate result of the collision, the nearby rock developed the numerous high-angle faults that now appear on the geologic map along the Mother Lode. The voluminous magmas of the batholith came into the country. Water moving down through the faults would have circulated close to or actually in–the magma, dissolving high-temperature gold com-pounds, and carrying them upward to precipitate the gold in fissures. In this manner, the Smartville Block, docking in the Jurassic, not only doubled the size of central California but created its Mother Lode.’ [p. 490
Ophiolite sequence:

Crustal sequence at mid-ocean ridges; visible in ophiolite.

“At its low end is
- peridotite, the rock of the mantle, tectonically altered* in several ways on departure from the spreading center. Above the mantle rock lie the cooled remains of the great magma chamber that released flowing red rock into the spreading center.
  - [layered gabbro] The chamber, in cooling, tends to form strata , as developing crystals settle within it like snow – olivine, plagioclase, pyroxene snow
    - [massive gabbro –> plagiogranite] but above these cumulate bands it becomes essentially a massive gabbro shading upward into plagiogranite as the magmatic juices chemically differentiate themselves in ways that relate to temperature. Just above the granites are
      - the sheeted dikes of diabase, which kept filling the rift between the diverging plates.
        
        [pillow lava] Above the sheeted dikes, where the fluid rock actually entered the sea, the suddenly chilled extrusions are piled high, like logs outside a sawmill. Because these extrusions have convex ends that bulge smoothly and resemble pillows, they are known in geology as pillow lavas.
        
        [sedimentary cherts, etc.] Above the pillows are the various sediments that have drifted downward through the deep sea: umbers, ochres, cherts, chalk. Unlike the rest of the crust-and-mantle package, the sediments may hint at the surrounding world.
        
        * Water that gets down through all this and into the mantle rock – at the spreading center or anywhere else – will change the nature and appearance of that rock. Through an alteration of minerals, the rock takes on a silky lustre and a very smooth texture, becomes fibrous, and develops color occasional streaks and spots of white, but mainly chrome green, myrtle green, Nile green, in patterned shapes within the mantle black. Because the patterns strongly suggest the skin of a snake, this rock has been known-for nearly six hundred years in the English language as serpentine.
        …
        From the mantle upward, the complete column of ocean-floor rock is collectively known in geology as an ophiolite. The generally consistent differences within it are the ophiolitic sequence.” [p. 480-481]
Auburn dam story. Also see: https://www.usbr.gov/history/ProjectHistories/Central%20Valley%20Project-Auburn%20Dam%20D2.pdf
The Smartville Block Accretionary Wedge.
“The Smartville Block pushed before it not only the limestone of Cool and the schists and serpentines of Aubum Dam but also the red-weathered phyllites and the argillites and cherts we had seen along the interstate as we descended toward Auburn. These and a great miscellany of additional rocks were Smartvlle’s melange, its accretionary prism – highly foliated, sheared, broken, disrupted. deformed — caught up in the Smartville suture, the docking of arc and continent.
There was, of course, a subduction zone – a trench – between the are and the continent as they drew together, and in the collision it disappeared. It was actually stuffed shut, according to present theory. First, ocean crust-and-mantle of the North American Plate went dovn the trench. Eventually, the continental rock itself reached the trench and jammed it, like a bagel in a toaster. ” [p. 484]
The tilt of the Smartsville Block. The ophiolite — the Smartsville Block – “now rests on California … listing 30° to the west. The ophiolite tilts more steeply than the Sierras. Therefore, as you climb the modern foothills, geologically you go down section, ever deeper into the former seafloor, from the spreading rift to the granites and gabbros of the magma chamber that fed it, and on to the cumulate layers of heavy crystals settled on the mantle at the … moho. On up the mountains, and farther down section, are the scattered serpentine derived from the mantle itself.” [p.
Below Timbuktu Bend on the Yuba river are green pillows of lava, undamaged, except for being rotated.
Metallic Ore Formation. Seawater descending through faults to the mantle dissolves metals and reprecipitates them on the seafloor; if this ends up on a continent, faults created during the suturing permits water to re-dissolve the metals and deposit them in cracks, forming veins.
Roadcuts in Smartville are full of bulging pillows.
Types of plate collisions:
Ocean crust colliding with continental crust can pry up something like the Andes.
Two plates of continental crust colliding can create the Himalayas.
Two plates of oceanic crust colliding generate island arcs.
You can use the diversity of marine life to predict whether there were many small continents (high diversity) or one large continent (lower diversity)
Ophiolites can be used to reconstruct plate tectonics from far in the past.
Ophiolite sequences are gradational, not sudden.
Ophiolite diversity. Ophiolites differ depending on whether they are from basins behind or in front of island arcs, or from spreading centers…
Two mohos. The petrological moho (olvine-to-mantle) and the seismic moho (where the olivine begins and speeds up seismic wave transition).
When plate motions change, transform faults may turn into subduction zones or spreading centers, or vice versa. “Across geologic time, subduction zones have come and gone quite often.”
Off the interstate near Dutch Flat: Feather River Periodite… (serpentine)… may be remains of older ophiolite. [p. 507]

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Cyprus. Cyprus ophiolite covered with chalk, an indication of its deep ocean origin. Some nice description: “the air was as cool as deep water, and about as still.” And “There were whole ridge lines of sheeted diabase, weathered out in silver blades, like thousands of playing cards in one standing deck…”
Moore describes finding a trace current, “a stream channel in the magma chamber.”
The rift valleys of spreading centers tend to break into blocks as they widen. [p. 514]
The rift valleys run around the world like stitching on a baseball. Each segment of a rift valley is about 40 miles long, and is connected to the next segment by a transform fault.
Cyprus means copper. Seawater descended through fissures into or close to the magma, where it picked up quantities of dissolved copper, and lesser amounts of mercury, manganese, tin, silver, a d gold, and then plumes of hot brine rose through the rock and precipitated it on the pillow lavas at the top of the ophiolitic sequence. Interestingly, once the copper (as copper sulphate) was on the surface, it could be reduced into metallic form by the resin from pines. …Lots more on ancient mining in Cyprus.
Macedonia. The Cyprus ophiolite has little mantle attached to it; to see a lot of mantle you have to go to Macedonia where 7 Km of it are exposed.
The Acropolis. The Acropolis, on which the parthenon stands, is a klipp (a remnant of a knappe, which is a body of rock that has been moved by faulting so that it is some distance from its origin. (How does it related to a horse?) More on this history of Greece, and places in Greece.
Crowne King, Arizona. Where Eldridge Moores grew up. History of his family. Mining and mines.
The Great Central Valley,. There are few things more singular. Fifty miles wide and 400 miles long, the valley is the floor of a swamp, and is extremely flat. It used to be that its micro-topography could be traces in the berms used to control water in rice paddies. There is a similarity here to the abyssal planes that extend from spreading centers, they are also flat, with only the most minute of inclines.
The Coast Range. These are fragmentary accumulations of marine clutter — part accretionary wedges scraped off the subjecting plate, and part igneous material produced by the effects of subduction.
The Sierra volcanos were the dying embers of the Farallon subduction, and Napa and coast range volcanism were the result of a new trench opening up as the smartville block clogged the old trench.
The Franciscan Melange…

As we go up the stream valley and arrive at the shore of Lake Berryessa, we pass through huge roadcuts of sedimentary rock whose bedding planes, originally horizontal, have been bent almost ninety degrees and are nearly vertical. Reaching for the sky in distinct un-rumpled stripes, the rock ends in hogbacks, jagged ridges. Cretaceous in age, these are the bottom layers of the Great Valley Sequence, bent high enough to resemble the bleaching ribs of a shipwreck. They are some of the strata that were folded against the Franciscan mélange when it rose (or was pushed) to the surface as the latest addition to the western end of the continent. In the heat and pressure of the Farallon Trench, the Franciscan sediments had been metamorphosed to varying extents, with the result that when they ultimately appeared on the surface they were miscellaneous and heterogeneous well beyond the brink of chaos. This lithic compote IS the essence of the Coast Ranges.
John McPhee, Assembling California, p. 545

The Melanage above Auburn

As we go up the stream valley and arrive at the shore of Lake Berryessa, we pass through huge roadcuts of sedimentary rock whose bedding planes, originally horizontal, have been bent almost ninety degrees and are nearly vertical. Reaching for the sky in distinct un-rumpled stripes, the rock ends in hogbacks, jagged ridges. Cretaceous in age, these are the bottom layers of the Great Valley Sequence, bent high enough to resemble the bleaching ribs of a shipwreck. They are some of the strata that were folded against the Franciscan mélange when it rose (or was pushed) to the surface as the latest addition to the western end of the continent. In the heat and pressure of the Farallon Trench, the Franciscan sediments had been metamorphosed to varying extents, with the result that when they ultimately appeared on the surface they were miscellaneous and heterogeneous well beyond the brink of chaos. This lithic compote IS the essence of the Coast Ranges.
John McPhee, Assembling California, p. 547

Pull-apart basins form when transverse faults stretch the earth, and relatively deep valleys open up like Napa Valley, Sonoma Valley, Ukiah Valley and the depression that holds Lake Berryessa
Submicroscopic gold is present in California and Nevada, and will soon make the US into the top gold-producing country, surpassing South Africa, by the turn of the 21st century.
Japan is moving towards North America at 1 cm/year — it will be part of the continent in 800 million years.

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