820 A TREATISE ON METAMOEPHISM. 



crystallized. Second, limestones in the belt of cementation may receive 

 important additions of silica even where they do not reach the surface but 

 are overlain by other rocks. It has been explained that the process of 

 carbonation of the silicates liberates colloidal silicic acid. This is taken 

 into solution, and passes downward into the belt of cementation. The 

 material transported to this belt, either from another part of the limestone 

 or from the silicates, is selectively precipitated as explained on pages 

 634-636, the limestone being simultaneously dissolved, and thus silicifica- 

 tion takes place. 



This process of segregation of silica, both within the belt of weathering 

 of the limestones and in the belt of cementation, is steadily cumulative. 

 Therefore it is possible for limestones which originally were not especially 

 siliceous to contain considerable amounts of silica. However, it is supposed 

 that usually those limestones which are heavily siliceous contained originally 

 a considerable quantity of silica as an organic precipitate. 



SILICATED MARBLES. 



The silicated marbles are products intermediate between marbles 

 and the silicate rocks. The processes by which they are formed are a 

 combination of those producing marble and those resulting in silication of 

 the carbonates. These processes will not be described in detail here, since 

 they are given under the immediately preceding and succeeding headings. 

 The conditions for their formation are intermediate between those ruling in 

 the production of evenly granular pure marble and those ruling in the 

 complete replacement of the carbon dioxide by silica. 



Like the marbles and the silicate rocks, the silicated marbles form in 

 the zone of anamorphism, and especially in connection with mechanical 

 action or igneous intrusion. The most abundant silicate minerals of the 

 marbles are pyroxene and amphibole, but many other silicates may form. 

 Of these, olivine, chondrodite, vesuvianite, tourmaline, and mica are the 

 most important. In any given case one or two silicates may be the 

 preponderant ones, and under such circumstances a more definite name 

 may be given to an individual rock or formation. For instance, if tremo- 

 lite is the dominant silicate it may be called a tremolitic marble (PI. V, B); 

 if diopside is the dominant silicate it may be called a diopsidic marble. If 

 two silicates occur together, as, for instance, diopside and olivine, the rock 



