DEVELOPMENT OF PORPHYRITiC CRYSTALS. 701 



orthosilicates ; staurolite and tourmaline are subsilicates. They are there- 

 fore unable to absorb a large amount of silica, and hence the explanation 

 of the abundant inclusions of quartz. The inclusion of quartz is finely 

 illustrated by some of the garnetiferous and staurolitic schists. In the 

 staurolites the quartz frequently seems as abundant as in the background, 

 but the micas and other minerals are usually absent, although they may be 

 abundant in the background. The garnet and staurolite have partly or 

 wholly absorbed the micas, chlorites, feldspars, and other minerals, which 

 are largely composed of the same elements as themselves, and have built 

 them into their bodies, thus making denser minerals. 



The evidence of this consists in the absence of inclusions of the iron- 

 bearing constituents of the schists in garnet and staurolite, and the presence 

 of abundant quartzose particles. While frequently the iron-bearing min- 

 erals extend without apparent diminution in amount to the garnets and 

 staurolites, in some cases around the garnets are aureoles or lenticular areas 

 of material which are markedly deficient in the iron-bearing minerals. 

 This I first observed at Bristol, Conn. (See PL II, B.) Here is a biotitic 

 granite blotched by white spots, consisting of quartz and feldspar with 

 no mica, the larger of them 2 cm. wide by 8 cm. long, in the cores of 

 which are large crystals of garnet The same thing is beautifully illus- 

 trated by the almandite in a mica-schist of Hampshire County, Mass. The 

 acid "minerals, such as quartz and feldspar^ outside or inside the garnets 

 which could not be absorbed have arrangements characteristic of the schists. 



The above occurrences give beautiful illustrations of the principle of 

 the development of the large heavy mineral particles from the material of 

 small and light ones. They also give positive evidence that the work is 

 done by solution and redeposition. The transportation of the material of a 

 mica flake to a garnet or staurolite must be done by solutions 



Commonly the porphyritic minerals do not show any orientation. But 

 in the schistose rocks containing porphyritic minerals it is usually true that 

 the minerals constituting the background have laminar forms, and often 

 similar crystallographic orientation. These minerals of the background 

 often stop abruptly at the junction with the garnets, staurolites, tourma- 

 lines, etc., without any deviation whatever or tendency to peripheral 

 arrangement about the porphyritic minerals. The micas may be seen with 

 perfect arrangements extending to the exact junction of the porphyritic 



