MINERALS OF GARNET GROUP. 301 



certain that various hydrous minerals furnish material for the formation of 

 garnet, and also the limestones and dolomites. As already noted, garnet 

 has a great variation in composition, and in a given case one of the pure 

 species mentioned, or a combination of the molecules of two or more of 

 them, will be formed which can be derived from the elements available. 

 For instance, from an impure limestone, calcium-aluminum garnet, grossu- 

 larite, is likely to form. In the magnesian rocks, magnesium-aluminum 

 garnet, pyrope, is likely, to be produced. In the impure aluminous 

 carbonates of calcium, magnesium, and iron, some combination of two or 

 more of the species grossularite, pyrope, almandite, and melanite is likely 

 to be produced. 



Garnet may be an original constituent of some of the igneous rocks. 

 If this be so, this source of garnet is comparatively insignificant, as it is 

 very rare indeed that garnet is found in an unaltered igneous rock. In 

 some of the little altered igneous rocks it is found in lithophysse, but the 

 garnets in this position are apparently the latest products of crystallization, 

 the conditions of their formation being analogous to those producing garnets 

 under the ordinary conditions of rock metamorphism. 



Considering the garnets individually, the following statements can be 

 made as to their occurrence: 



Grossularite is especially common in the marbles, where it is frequently 

 associated with vesuvianite, wollastonite, diopside, etc. It also occurs in 

 the calcareous schists and gneisses, especially in the calcareous siliceous 

 rocks, such as calcareous quartzites and calcareous novaculites. Grossularite 

 also is associated with common garnet in other schists and gneisses. It is 

 recorded as being derived from melilite and gehlenite. 



Pyrope, the magnesium garnet, as would be expected, is especially 

 prevalent in peridotites and their derivatives, such as serpentine and talc, 

 since these rocks are rich in magnesium. It also occurs in some basalts. 



Almandite, one of the most widespread of the pure garnets, occurs in 

 granites, schists, gneisses, and granulites, and thus is present in both 

 feldspathic and feldspar-free schists. Almandite is also known in certain 

 andesites. It rarely has crystalline forms. 



Spessartite occurs in large and small grains in contact rocks, in 

 porphyritic crystals of large size in quartzites, and is abundant in certain 

 whetstone-schists. With topaz, it is known in lithophysse in rhyolite. 



