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University of California Publications in Geology [Vol. ll 



It indicates different stages of a process of crystallization in a solid amorphous 

 mass. If this be granted there seems to be no good ground for doubting that in 

 general the holocrystalline cherts, or jaspers, were originally amorphous silica, 

 and that they have reached their present form by a process of crystallization quite 

 analagous to that of devitrification in volcanic rocks. 



Diller has described similar sections from some of the radiolarian 

 cherts of the Port Orford quadrangle in Oregon, in which the matrix 

 contains much amorphous silica. He, also, regards this as proof that 

 the cherts were originally composed of amorphous silica which has 

 undergone a process of crystallization. 



It is extremely interesting to find that the cherts which show this 

 isotropic matrix are rather hard — ranging from 6 to 7. The sugges- 

 tion is strong that we have here a new mineral form of silica. This 

 was noted by Professor Lawson 13 in his report on the Geology of the 

 San Francisco Peninsula : 



What is the mineralogical character of the amorphous silica? It is clearly 

 not any one of the numerous cryptocrystalline varieties of silica. It is perfectly 

 isotropic. If we refer to the books we find that opal is the only variety of silica 

 which is strictly amorphous. But this differs from opal. A fragment of chert 

 that showed a large proportion of isotropic base was found to scratch quartz dis- 

 tinctly with comparative ease. It has a specific gravity of 2.628, and although it 

 yielded a little water in the closed tube, the amount of water was notably less 

 than that derived from a corresponding quantity of common opal. We thus seem 

 to have a condition of slightly hydrous amorphous silica much harder and much 

 heavier than opal. 



The present writer found a somewhat similar variety of silica in 

 examining a specimen of chert brought from lake beds in the Great 

 Basin. This chert was hard enough to scratch steel but powdered 

 fragments of it were perfectly isotropic. The index of refraction 

 was about 1.535. 



The transparent areas, which represent the radiolaria, are usually 

 free from iron oxide, and are the casts of originally hollow skeletons. 

 These areas often show the external form of the organism, and in 

 many instances portions of the delicate spines are preserved. In a 

 few cases nearly perfect skeletons are found (plate 30). In looking 

 over a suite of slides, one may see all degrees of preservation from 

 nearly perfect fossils down to those in which the outlines can be 

 made out with difficulty, and in which only the remnants of spines 

 and external forms are preserved. In the extreme cases only irregu- 



13 15th Ann. Eep. U. S. Geol. Surv., p. 423, 1895. 



