﻿ASBESTOS AND MANGANESE DEPOSITS. 159 



the similarity existing between the phenomena presented here and those 

 observed in the San Francisco rocks, and also because of the excellence 

 of his petrographic descriptions: 



"When a suitable series of these cherts is viewed in thin section under the 

 microscope a gradation may be observed from those which are composed almost 

 wholly of amorphous or isotropic silica to those which are holocrvstalline aggre- 

 gates of quartz granules. In. the most isotropic sections there are, however, 

 numerous minute scattered points in the field, which polarize light. These can 

 not be separated in any sharp way by the highest powers from the isotropic base. 

 They are not inclusions, but centers of incipient crystallization in the amorphous 

 rock. They correspond to the products of devitrification in glass. In other 

 slides these centers of crystallization are much more thickly crowded, and definite 

 areas composed of interlocking granules of quartz appear, interlocking, also, with 

 the isotropic base. The actual boundaries of these areas can be made out only 

 with difficulty and uncertainty, owing to the fact that the quartz granules are 

 characterized by a molecular tension, which results in an undulatory extinction 

 as the stage is resolved between crossed nicols. In still other slides these areas 

 coalesce and the proportion of amorphous base to the whole rock becomes very 

 small * * ». 



"Scattered through the slides of these cherts, whether they be amorphous, 

 semicrvstalline, or holocrvstalline. there may usually be observed, in ordinary 

 light, circular or oval clear spaces or clear rings free from pigment. Between 

 crossed nicols these clear spaces are seen to be occupied by chalcedony. They are 

 the casts of Badiolaria, and occasionally remnants of the spines and latticework 

 may lie detected. • These areas and rings are usually more sharply defined in the 

 amorphous cherts, and are somewhat indefinite in outline, yet distinct as areas, 

 in the holocrvstalline varieties. In thin section they are most readily observed in 

 the red cherts, by reason of the contrast which they make with the pigmented 

 matrix. 



******* 



"'It thus seems to the writer that the bulk of the silica can not be proved to 

 be the extremely altered debris of Radiolaria. The direct petrographies] sug- 

 gestion is that they are chemical precipitates. If now we accept this hypothesis, 

 it becomes apparent that there are three possible sources for the silica so preci- 

 pitated, viz: (li Siliceous springs in the bottom of the ocean, similar to those 

 well known in volcanic regions: i,2 ) radiolarian and other siliceous remains, which 

 may have become entirely dissolved in sea". water; and (3) volcanic ejectamenta, 

 which may have become similarly dissolved." 



I believe that these clear spaces in the Iloeos Xorte slides do not 

 represent individual casts, for I find no trace of "latticework** and but 

 little to compare with the spines found in the California cherts : in fart. 

 I think these clear, circular and oval areas represent pores in the tests 

 of such Xasselarians as Podocrytis, Bofhryocampe, etc.: as yet I have 

 been unable to make any specific determinations from these slides. 



The Bangui Sandstone. — On the slopes of Monte Inmenso I found a 

 very coarse-grained, friable sandstone the beds of which on the average 

 measure "2 feet in thickness, dipping at rather high angles, from 45 c 

 55711 2 



