372 University of California Publications in Geology [Vol. ll 



source. It is hardly possible that beds so much alike and within a 

 few feet of one another should have undergone such radical differences 

 in crystallization of the original silica, either in an ooze on the sea 

 floor, or in the later alteration of the rocks. 



Evidence is also found in the fact that different portions of the 

 same bed, or different portions of the same hand specimen, or even of 

 the same thin section, show different numbers of radiolaria. One 

 specimen, of red siliceous chert, in the possession of the writer brings 

 out this fact in a striking way. The specimen shows three zones run- 

 ning parallel to the bedding. The upper zone is characterized by a 

 large number of radiolarian skeletons, which is also true of the lower 

 zone. Through the center of the specimen there runs another zone, 

 that, though in other respects exactly like the upper and lower zones, 

 contains only a very few radiolaria. Such differences cannot be due 

 to difference in the degree of crystallization and reworking which has 

 affected the rock. Similar specimens are common, and they indicate 

 that the proportion of radiolaria to the total amount of silica accumu- 

 lating, varied from time to time during the deposition. 



A similar, banded distribution of radiolaria was noted in a thin 

 section of a kieselschiefer from Lautenthal in the Harz Mountains. 

 In this thin section there are two bands. One band contains numerous 

 radiolaria, showing a few of the details of the outlines and the spines. 

 These are represented by areas of clear silica, which between crossed 

 nicols are seen to be made up of spherules of radiating fibers of chal- 

 cedony, or of cryptocrystalline silica. These organisms are embedded 

 in a matrix of dark silica which appears in large part to be non-polar- 

 izing, with here and there a spot of light. The other band shows no 

 radiolaria. It consists of silica clouded in a streaky manner by 

 masses of pigment. This band is almost wholly composed of non- 

 polarizing silica. Between crossed nicols, with the gypsum plate, no 

 reaction can be observed, as the stage is turned, save in a few isolated 

 patches .which appear to have undergone crystallization. 



Possibility that Matrix may be Derived from other Organisms. — 

 If we abandon the idea that all the silica in the cherts was originally 

 in the form of radiolarian skeletons, we may have yet another possi- 

 bility. We may regard the cherts as originally a mixed ooze, contain- 

 ing dther organisms than radiolaria. It may be possible, for exam- 

 ple, that some part of the silica was originally in the form of diatoms. 

 It is well known that some radiolarian oozes in the neighborhood of 

 the Philippines contain such a large proportion of diatoms that they 



