SILICATED MARBLES. 821 



may be called a diopsidic and olivinitic marble. In a similar manner, other 

 appropriate names may be given to individual formations, more accurately 

 defining their character than the general term silicated marbles. 



Of the two most prevalent groups of minerals, pyroxene and amphi- 

 bole, calcium is more abundant in the former. Therefore from the nearly 

 pure calcium carbonate rocks the pyroxenes are likely to develop, rather 

 than the amphiboles. Thus from the rocks which have not been strongly 

 dolomitized wollastonite and diopside form, rather than tremolite, because 

 the latter mineral requires a larger proportion of magnesium. Where the 

 rocks have been dolomitized before silication the amphibole tremolite is 

 especially likely to develop. 



In the calcareous rocks in which there is a considerable amount of 

 fragmental material mingled, and consequently aluminum is plentiful, the 

 aluminous pyroxenes and amphiboles are likely to form. 



In so far as the process of silication takes place by the combination of 

 solid silica with the carbonate, and the liberated carbon dioxide escapes, 

 the volume of the rock is decreased. The decrease in volume varies from 

 a very small percentage to very considerable amounts as they approach 

 the silicated rocks, which have a decrease in volume varying from 20 to 40 

 per cent. 



The silicated marbles grade into the pure marbles on the one hand and 

 into the silicate rocks on the other. The term silicated marble is properly 

 applied to rocks which range from those in which the silicate minerals are 

 unimportant to those in which the carbonates are subordinate. Those rocks 

 in which the silicates are not abundant should be classified with the 

 marbles, and those in which the carbonates are not abundant should be 

 classified with the silicate rocks Thus limited, the silicated marbles occur 

 at very numerous localities among the rocks profoundly metamorphosed 

 under deep-seated conditions. This state of affairs naturally exists to a 

 greater extent among the old than among the new formations, and therefore 

 this class of rocks is especially abundant among- the Paleozoic and pre- 

 Paleozoic sedimentary rocks. Illustrative localities of the silicated marbles 

 occur in almost every country. The best representatives in this country 

 are found in the pre-Cambrian formations of Lake Superior and Canada 

 and in the Paleozoic formations of the Appalachian region. 



