26 



University of California. 



Vol. i. 



tropic substance, which can, on the whole, only be regarded as an 

 amorphous form of silica. Such silica can, farther, it seems to us, 

 only be regarded as of the nature of an original chemical deposit. 

 The question is whether the silica was derived from organic 

 remains or from volcanic glass. Whichever form the silica had 

 originally, there seems no escape from the conclusion that the sea 

 water has affected it on a wholesale scale as a chemical reagent, dis- 

 solving it and reprecipitating it as pulverent or gelatinous silica. 

 The chalky formations probably represent the pulverent deposit 

 and the opaline beds the gelatinous. Are there any criteria for dis- 

 tinguishing whether this homogeneous isotropic ground-mass is 

 derived from volcanic glass dissolved in sea \vater, or from organic 

 remains affected in the same way? So far as we can discover, there 

 are no direct criteria available. There are, however, indirect con- 

 siderations which, though not demonstrative, are weighty, and favor 

 the volcanic hypothesis. These may be thus summarized : — 



1. Siliceous organic remains are not common throughout the 

 great mass of the formation. Calcareous organisms were very abun- 

 dant, but are now for the most part leached out, leaving definite 

 cavities or moulds. Siliceous organisms, like those of the Mont- 

 erey infusorial beds, appear to be limited to certain definite horizons 

 of small vertical extent. No siliceous organisms have been found 

 in our examination of the shale near Carmelo Bay. 



2. The chemical analysis corresponds approximately to that of 

 a very acid soda rhyolite, and not to that of an organic deposit. 



3. The angular fragments of mineral scattered through the 

 ground-mass are those of an acid volcanic rock, and they are not 

 water worn. 



If this hypothesis of the volcanic origin of the Monterey series 

 should stand the test of future research, our current ideas regard- 

 ing the Miocene of the California coast would have to undergo an 

 important modification. Instead of being an organic deposit of ex- 

 cessively slow growth, we should have to regard it as an ash of 

 comparatively rapid accumulation, with calcareous organic remains 

 far exceeding the siliceous. 



Although the volcanic hypothesis is not here established, it 

 seems desirable to advance it, if only to the end that the old and 



