1918] Davis: The Badiolarian Cherts of the Franciscan Group 255 



too much changed by crystallization of the matrix, they show dis- 

 tinctly the spines, characteristic outlines, and in a few cases, even the 

 internal lattice structure of these organisms. 



In most instances, the matrix of the rock consists of a micro- 

 granular aggregate of chalcedonic silica. Each separate grain shows 

 a fibrous wavy extinction, the direction of which is different from 

 that in adjoining grains. The grains appear to interlock in a fine 

 mosaic. Lying between these grains, and included in them, are numer- 

 ous specks of red iron oxide, which give the matrix a deep red color. 

 In some instances this material is so abundant that the matrix is 

 practically opaque and appears as a dark red translucent mass in 

 which no detail is observable. Occasionally the matrix consists of 

 quartz grains without fibrous crystallization. While the matrix, in 

 most of the thin sections examined by the writer, consists entirely of 

 anisotropic silica of the kind described above, there were a few in 

 which the matrix is not ciystalline but amorphous. One section 

 examined was almost entirely isotropic, showing only here and there 

 a minute grain of polarizing material. Several others showed con- 

 siderable irregular areas of polarizing silica with residual patches of 

 isotropic material. 



These facts, with their probable meaning, have been well described 

 by Professor Lawson, 12 in the following words: 



If a selected series of thin sections of these cherts is viewed under the micro- 

 scope they present a gradation from those that are composed almost wholly of 

 amorphous or isotropic silica to those that are a holocrystalline aggregate of 

 quartz granules. The most isotropic sections, however, exhibit numerous minute 

 scattered points that polarize light and that cannot be sharply separated, even by 

 the highest powers, from the isotropic base. These points are not inclusions ; they 

 are centers of incipient crystallization in the amorphous rock, corresponding to 

 the products of devitrification in glass. In other sections these centers of crystal- 

 lization are much more thickly crowded and well-defined areas composed of inter- 

 locking granules of quartz appear, interlocking also with the isotropic base. The 

 actual boundaries of these areas can be made out only with difficulty and uncer- 

 tainty, owing to the fact that the quartz grains are under molecular strain, which 

 produces undulatory extinction as the stage is revolved between crossed nicols. 

 In still other sections these areas coalesce and the proportion of amorphous base 

 to the whole rock is very small. Finally, some sections show a holocrystalline 

 aggregate of interlocking quartz grains. Most of the grains are under molecular 

 strain, as is shown by undulatory extinction, .and somewhat resemble chalcedony. 

 The discrimination between the amorphous -and the crystalline silica is easy in 

 those varieties of the rock that contain little iron-ore pigment but becomes more 

 difficult as the abundance of the obscuring pigment increases. The gradation 

 thus observed in a series of thin sections prepared from specimens taken at ran- 

 dom seems clearly to be a gradation in time and not merely a gradation in space. 



12 Lawson, San Francisco Folio, op. cit., p. 5. 



