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University of California. 



[Vol. 2. 



cisco, and is, indeed, of extensive occurrence in the Coast Ranges 

 generally. 



In the older writings on Californian geology it was referred to 

 as the San Francisco sandstone, from its prevalence at the city of 

 San Francisco. In later years it has been recognized as a constitu- 

 ent member of a series of formations to which the name Franciscan 

 has been given. Other formations are characteristically associated 

 with this massive sandstone to make up the Franciscan series. 

 Among the most interesting of these are certain rhythmically 

 bedded, flinty rocks, or cherts, which are known to geologists as 

 radiolarian cherts, from the occurrence in them of numerous fossil 

 remains of the minute tests of radiolaria, or the casts of such tests. 

 These radiolaria were marine animals which secreted a silicious 

 skeleton or test of great beauty and complexity of structure, which 

 sank on the death of the animal and became entombed in a sili- 

 cious ooze then accumulating on the sea bottom. Their fossil 

 remains may be readily seen by examining a smooth surface of 

 almost any fragment of the chert with a pocket lens, and the 

 observation is made much easier by wetting the surface. In thin 

 section of the chert they appear on the stage of the microscope 

 usually as discrete rounded areas of clear chalcedony, about .25 

 mm. in diameter, in a matrix of microcrystalline quartz, and quite 

 commonly remnants of the original form of the test may be 

 detected. The forms of these tests are very varied, and each 

 particular species had its own peculiarity of skeletal architecture; 

 and it is upon this basis that they are classified. Numerous genera 

 and species are represented in the radiolarian cherts of the Francis- 

 can series, but the distribution of these creatures in geological time 

 is not yet sufficiently well known to enable us to determine the age 

 of the rocks in which they occur, except in a very broad way. 

 That is to say, although these fossils indicate that the rocks are of 

 Mesozoic age, the subdivision of the Mesozoic, to which they 

 belong, can not be indicated with certainty. The rhythmically thin 

 bedding of these radiolarian cherts is one of their most interesting 

 features, and the one which is most difficult to explain. The 

 absence of detrital material in these cherts of course indicates that 

 they were laid down on a sea bottom deep enough to be far 



