344 University of California Publications in Geology [Vol. ll 



material, yet the feldspar is nowhere in crystals or grains, and in much of the 

 rock must be sparingly present. We may distinguish it as prasoid rock, for it 

 is abundant wherever the talcose formation occurs. . . . 



A light greenish variety of this rock, near San Francisco, is associated with 

 red and yellow jasper; some hills consist wholly of the latter material, while 

 in others both the green and red rocks are associated, showing by their grada- 

 tions the close relations between the jasper and the prase rock. 



A variety resembling bloodstone is also met with, at times; it has a dark 

 green color and jasper-like fracture, though no specimens were seen with the 

 red spots of true bloodstone. 



From the transitions here pointed out, it appears that the jasper and prase 

 rocks are closely connected with the talcose series; and that the translucent 

 prases and bloodstones here found are only varieties of its condition. 



Later in the chapter he describes the cherts more fully : 



The structure of the jaspery rock of San Francisco is worthy of description. 

 The green, red and yellow varieties occur in the same vicinity. They form a 

 series of layers, averaging two inches in thickness, and varying from half an 

 inch to four inches. The layers are very distinct, and are partially separated 

 by open seams, and on the front of bluffs or ledges the rock has consequently 

 a riband-like appearance. The layers often coalesce and subdivide without 

 regularity, though uniformly parallel. They are frequently twisted, and thus 

 change, at short intervals, from a vertical position to a dip of twenty degrees. 

 The colors, red and yellow, are often mingled, and sometimes appear as par- 

 allel bands. In some instances the surface is red, while the rock is yellow 

 beneath: this has resulted from the burning of a tree on the spot; for by heat 

 the yellow variety readily changes to red. A small specimen of the green 

 variety had an agate-like structure, as if it had been formed from an aqueous 

 solution. 



The next reference to these rocks is found in a booklet by Philip 

 T. Tyson, 103 published in 1851. He appears to have believed that 

 they were metamorphosed sediments of some sort. The following 

 appears in his descriptions of the region around Bodega : 



The northwestern end of this small vale is abruptly terminated by steep 

 acclivities, whose rocks mainly consist of several varieties of compact quartz 

 such as chert, jasper, etc. There is every shade of white and gray to nearly 

 black, as well as yellow, red, and brown, each color continuous in separate 

 layers. It is evident that they were originally deposited in strata; and, besides 

 having been changed in character by heat, were contorted and twisted in a 

 remarkable manner anterior to, or during the period of their elevation. 



John B. Trask, in 18 53, 104 described the cherts at Presidio Point 

 in the following words : 



103 Geology and Industrial Eesources of California, Baltimore, 1851. 



104 Geology of Sierra Nevada and Coast Ranges, California Sen. Exec. Doe. 

 no. 9, p. 5, Sacramento, 1853. 



