1918] Davis: The Radiolarian Cherts of the Franciscan Group 279 



DIATOMACEOUS EARTHS 



The diatomaeeous earth, popularly called "chalk rock," is the 

 most characteristic of the rocks of the Monterey group. It is com- 

 posed largely of the frustules of diatoms. The purer varieties are 

 generally rather soft, sometimes powdery, and are white, cream or 

 light gray in color. Generally the diatomaeeous earth is massive and 

 shows no well denned lamination, but occasionally it shows distinct 

 thin bedding which rarely is of " paper thinness. ' ' Under the micro- 

 scope the skeletons of diatoms are seen in a well preserved condition. 

 In addition to the well preserved diatoms there is a considerable 

 amount of material which seems to represent the finely divided debris 

 of broken diatom frustules. 



DIATOMACEOUS SHALES 



The diatomaeeous shales represent all gradations between ordi- 

 nary shales and the pure diatomaeeous earths. Under the microscope 

 the shales are seen to consist of the skeletons of diatoms and irregular 

 quartz grains scattered through a very fine grained matrix, the nature 

 of which cannot always be determined by the microscope. Some of 

 the finer materials of these shales may consist of the finely broken 

 frustules of diatoms, but argillaceous matter is also present in vari- 

 able amounts, as is indicated by chemical analysis (p. 290). They 

 often contain notable amounts of lime and magnesia as carbonates. 

 By increase in these different constituents, the - diatomaeeous shales 

 pass into calcareous or argillaceous rocks. 



The diatoms are not the only organic remains in these rocks. A 

 few sponge spicules and radiolarian skeletons are also found. Locally 

 the diatomaeeous shales contain many radiolarian skeletons, in an 

 excellent state of preservation. The less siliceous shales also contain 

 numerous scales of fish, and occasionally the bones of fish are found. 

 In a few instances bones of whales or other marine animals have been 

 discovered. 



Shells of foraminifera are characteristic of many of the rocks of 

 the Monterey group and are found abundantly in many beds in a 

 state of excellent preservation. Very often also they have been dis- 

 solved out, leaving characteristic moulds. 



These diatomaeeous shales have been regarded as the source of a 

 large part of the petroleum of the California Coast Ranges; it is be- 

 lieved that the petroleum came from the organic matter of the dia- 

 toms whose remains are found in them. 



