MINERALS PRODUCED IN CONNECTION WITH INTRUSION. 7 1 7 



to it. All of the above relations may obtain about a single batholith, as in 

 the case of the Black Hills granite already mentioned. At places where 

 the new cleavage develops at right angles to the old cleavage, the older 

 may be completely obliterated near the batholith. Farther away from 

 it the old and new cleavage may both be present and intersect each other, 

 and still farther away the old cleavage may be dominant. a 



THE MINERALS. 



The minerals which form depend upon the intensity of the metamor- 

 phism. To illustrate, closest to the Black Hills granite batholith is a gneiss 

 containing abundant quartz, mica, feldspar, garnet, staurolite, hornblende, 

 and tourmaline. Farther from the granite is a schist containing quartz 

 and mica, comprising- both muscovite and biotite, with staurolite and garnet. 

 Farther out is a schist in which the staurolite is absent and garnet is present. 

 Still farther away is an ordinary slate in which there are small flakes of 

 mica, but in which there are no garnets or staurolite. Similar illustrations 

 are furnished by the batholiths of Massachusetts, described by Emerson; 6 

 but these have, in addition to the minerals above mentioned adjacent to 

 the granite, cyanite; somewhat farther out in the schists, sillimanite; and 

 still more remote in the ordinary slate, andalusite. The three minerals 

 mentioned furnish a particularly good illustration of thp metamorphic 

 effect with reference to position, since they have precisely the same chem- 

 ical composition. Also these minerals furnish a perfect illustration of the 

 principle, expounded in Chapter IV (pp. 182-186), that in proportion as the 

 metamorphic effect is jDrofound and the pressure great, minerals of high 

 specific gravity develop. The specific gravities of andalusite, sillimanite, 

 and cyanite are, respectively, 3.18, 3.235, and 3.615, and this is their order 

 observed in approaching- the intrusives. 



In a given case the different mineral zones are not sharply defined, 

 but grade into one another. Moreover, the most intensely metamor- 

 phosed area may contain all the minerals developed by the feebler meta- 

 morphic processes This may be explained on the supposition that as 

 the metamorphic wave passes away from the central batholith the condi- 

 tions were first those of mild metamorphism, later those of moderate meta- 

 morphism, and finally those of severe metamorphism. Also adjacent to the 



« Van Hise, cit., pp. 232-234. * Emerson, cit, pp. 559-561. 



