46 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 30 Ser. 



and difficult to be undertaken, and moreover, the material 

 at hand, though sufficient to show the general character of 

 the organisms, would not be enough for determination of 

 particular forms. 



"No. 90. This soft, white, earthy rock is essentially 

 diatomic in character. Both in section and in powder it is 

 seen to consist of a mass of heterogeneously mingled frag- 

 ments of diatom frustules, with a small proportion of com- 

 plete forms. By far the larger mass of the rock is formed 

 by the broken up and disintegrated particles of the diatoms, 

 and the smallest and finest portions recognizable under the 

 microscope are clearly organic debris. Coscinodiscus appears 

 to be the predominant genus. Detached sponge spicules are 

 fairly numerous. They are principally pin-shaped and 

 styliform; also a few simple fusiform rods occur belonging 

 to the Monactinellid division of siliceous sponges. The 

 Tetractinellid sponges are represented by a few fragmentary 

 trifid spicules and globate forms. Only one or two some- 

 what doubtful fragments of Radiolaria- were noticed, and 

 these organisms must have been very sparsely present, for 

 their structures are stouter and more capable of preservation 

 than the diatoms. The silica of these organisms — diatoms 

 and sponges — appears to be unaltered in the fossilization — 

 it retains the same glassy aspect as in recent examples. In 

 addition to the siliceous organisms, Foraminifera are like- 

 wise present, and they yet retain the calcareous structure of 

 their walls, though hardly so well preserved as in the case 

 of the siliceous fossils. A rather larafe form of Textiilaria 

 is the most common of the Foraminifera. It is to these 

 organisms that the calcareous portions of the rock are due. 

 The rock is very finely laminated, showing a series of well 

 marked undisturbed layers of organic remains in which are 

 scattered some minute angular chips of minerals here and 

 there. 



"No. 118. A whitish, comparatively soft, earthy rock. 

 Examined both in section and in powder. Very similar to 

 the preceding in consisting nearly wholly of diatoms and 

 diatomic debris. Coscinodiscus is very numerous; some 



