ESTIMATES OF TIME 831 



900 feet thick in the San Francisco region and the latter about 530 feet. 

 They are preceded and succeeded by thick sandstone formations and are 

 separated by the Marin sandstone, 1,000 feet in thickness. The two chert 

 formations consist of sheets of chert alternating with partings of shale. 

 Lawson describes these formations and their conditions of deposition as 

 follows : 



"The thickness of the sheets of chert in the typical sections generally ranges 

 from about 1 to 3 or 4 inches, aA^eraging perhaps 2 or 3 inches. Some beds are 

 much thicker, but the sections of the chert nevertheless in general show thin 

 and even bedding. The shaly partings between these sheets usually range 

 from about one-eighth to one-half inch in thickness, but many of them are 

 mere films. As the formations are in some places exposed in sections that are 

 several hundred feet thick, they present the remarkable phenomenon of an 

 alternation of thousands of layers of chert, with as many layers of shale. In 

 the common red phase of the formations the regularity of this thin-sheeted 

 stratification is amazing. In other phases, in which red iron oxide is not so 

 abundant, the regularity is much less marked and the sheets assume lenticular 

 forms. " 



"On a smooth surface of almost any specimen of these cherts a lens will 

 reveal minute round or oval dark, hyaline, or whitish dots. These dots, which 

 are scattered through the rock, prove on microscopic examination to be the 

 remains of Radiolaria, the characteristic fossils of these formations. The 

 Radiolaria are minute animals that thrive in sea-water and secrete siliceous 

 skeletons of very complex structure. These skeletons evidently accumulated 

 in great numbers on the floor of the sea w^hile the radiolarian cherts were 

 being deposited and thus contributed to their formation. As a rule, they are 

 sporadically embedded in the siliceous matrix above described, but in some 

 places they are so closely crowded as to constitute the greater part of the 

 chert. Where the Radiolaria are scantily distributed through the chert it is 

 uncertain whether or not the matrix also is derived from these organisms, and 

 the alternative hypothesis that it was formed by the purely chemical precipi- 

 tation of silica, supplied possibly by submarine springs, is worthy of considera- 

 tion. If the silica is wholly of organic origin it must have been dissolved and 

 reprecipitated in its present form as an ooze on the sea-bottom. 



"As subsidence proceeded the detrital material washed from the receding 

 shore again failed to reach the region and organic agencies once more resumed 

 sway. This time, however, calcareous organisms were replaced by those which 

 secrete silica from the sea-water, so that the sea-bottom was covere<l with 

 radiolarian ooze, which eventually consolidated as the Sausalito chert. The 

 rhythmical oscillation of conditions which produced the remarkable alterna- 

 tion of layers of chert and shale in this formation has not yet been explained, 

 but was probably due to alternating conditions in the sea-water which affected 

 or interrupted the swarming of radiolarian life. The accumulation of this 

 radiolarian ooze was stopped by a recurrence of the shallowing of the sea and 

 the return of the shoi-e to n line sufficiently near to insure the deposition of 



"^2 Loc. cit., pp. 5, 6. 



LXII— Bull. Geol. Soc. Am., Vol. 28, 1916 



