L.AWSON "1 



I'alacheJ 



The Berkeley Hills. 



445 



trolled the accumulation of such rocks. It must be confessed, 

 however, that no very satisfactory hypothesis has yet been formu- 

 lated. Of course, various extreme possibilities occur to the mind, 

 but we have little or no check upon them. For example, the 

 rhythm might be due to an annual rotation of conditions. This 

 would mean that each pair of layers in the series, one of chert and 

 one of shale,' is the result of one year's accumulation. The average 

 thickness of such pairs may be placed at three inches. This would 

 mean that the entire 1,000 feet had accumulated in 4,000 years. 

 This estimate shocks our current conceptions of the rate of accu- 

 mulation of deposits which are chiefly non-detrital. It involves 

 moreover a startling rate of abstraction of silica from sea water, 

 whether the abstraction be effected by organic agencies or by 

 direct chemical precipitation. If we assume that this particular 

 basin in which the Coast Range bituminous shales were deposited 

 had an area of 1,000x500 miles, the annual deposit would amount 

 to about 100,000 mile-feet of silica in the form in which we now 

 find it, which would be about 200 times, by rough estimate, the 

 total amount of silica contributed annually to the ocean by all the 

 rivers of the world. We have been unable to find figures for the 

 proportion of silica contained in sea water, but the rate of abstrac- 

 tion In this limited area is so much in excess of the supply to the 

 entire ocean that the possibility of the rhythm being due to annual 

 rotation of conditions seems to find little support in the computa- 

 tion. Another possibility is that there may have been some 

 rhythm in the swarming of silica-secreting organisms in the sea 

 water. This, however, in the present state of our knowledge is 

 pure conjecture and can not be regarded as a scientific hypothesis 

 till we have some information which would support it. 



Still another possibility is th it there may have been some 

 rhythmical supply of silica from purely inorganic sources, such as 

 submarine solfataric springs. The only fact that can be adduced 

 in favor of this possibility is that in the lower part of the Mon- 

 terey series in Southern California there is a great thickness of 

 volcanic ashes, and that in some other portions of the series 

 volcanic material forms a moderate but distinct!) recognizable 

 constituent in the bituminous shales. This, however, contributes 



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