818 A TREATISE ON METAMORPHISM. 



The question now arises why the chert occurs so generally segregated 

 in nodules, thin layers, and bands in the limestone. The silica as deposited 

 by organisms is in the amorphous, and therefore readily soluble, form. It 

 has been explained on pages 120-123 that during the rearrangement of 

 minerals by solution and deposition there is a marked tendency for those of 

 the same kind to segregate, and this tendency is strong in proportion as the 

 minerals are readily soluble. At a point where there is more than an average 

 amount of silica, especially large grains of quartz, this silica draws to it 

 silica from the surrounding rocks through the solutions, under the principle 

 expounded on pages 74-76, that large masses and grains grow at the 

 expense of smaller ones. In this principle we have the explanation not 

 only of the segregation, but of the lenticular character which is so charac- 

 teristic of the masses. The principle under which larger masses or grains 

 grow at the expense of smaller ones is that the change tends to give the 

 total mass the smallest surface area. This would be ideally accomplished 

 by the spherical form of the masses; but since openings are numerous and 

 circulation is comparatively easy' along the beds, the lenticular masses, 

 especially those which are more than a few centimeters in diameter, have a 

 greater lateral than vertical extent. But in proportion as the chert masses 

 are small they are likely to approach the spherical form, and this gives proof 

 of the correctness of the principle. It is therefore believed that the segre- 

 gation and form of nodules and thin lenticular layers of chert in limestones 

 are adequately explained by concentration through solution and deposition. 



Where, however, there are bands of chert of considerable thickness, 

 so as to constitute definite members of a formation, and especially where 

 there are formations of chert, the above explanation is not adequate. It is 

 probable that many such deposits were originally dominantly siliceous. 

 The principle of the natural groupings of animals of like kind into a 

 colony is well known. Where there was a colony of silica-secreting 

 organisms a major band or formation of silica would form. Modern deposits 

 composed of nearly pure silica formed by siliceous organisms show how 

 lenticular bands of chert may have originated. For instance, the well- 

 known diatom deposit of chert used as tripoli at Richmond, Va., is about 

 10 meters thick. a Such deposits may have been enlarged by segregations 



« Merrill, George P., The nonmetallic minerals of the U. S. National Museum: Kept. Smithsonian 

 Inst, for 1899, Washington, 1901, p. 219. 



