DEVELOPMENT OF ACCESSORY MINERALS. 221 



direction and also at right angles to it. Contained abundantly are very 

 numerous particles of iron oxide — hematite, — many of which have crystal 

 outlines. If it were not that this inclusion chances to be present there 

 would be nothing in the thin sections, so far as one can see, to show the 

 genesis of these rocks. When they are examined closely it is seen that 

 while this material is contained in the most of the feldspars, in the mica 

 and in the smaller particles of quartz, it is only contained in the exterior 

 portions of the larger particles of the latter mineral. These show round 

 or oval cores which are perfectly free from this inclusion. In these rocks, 

 as in the thoroughly crystalline mica-schists, the cores of quartz stand as 

 the only representatives of the original detritus. All other materials have 

 recrystallized. That the major part of the feldspar is really authigenic 

 rather than remnants of clastic particles is shown by its freshness, by its 

 inclusions, and by the fact that in one case its twin lamellae, probably in 

 obedience to pressure, uniformly follow the direction of schistosity. 



Garnet, Staurolite, and Tourmaline. — In all the foregoing rocks are found 

 various accessory minerals, the chief of which are garnet, tourmaline, and 

 staurolite, although Newton says tourmaline occurs only in the granite.* 

 Within the quartzites, conglomerates, and greywackes these minerals are 

 relatively unimportant. They increase in abundance as the rocks become 

 more crystalline, and in some cases they become principal constituents. 

 Garnet is much more widespread than the other two, although the amount 

 of tourmaline and staurolite present is very considerable. These minerals 

 all have the characteristics usual when occurring in crystalline schists, and 

 they will therefore not be described in detail. As much has not been made 

 out as to their manner of growth as could be wished. There is little doubt 

 that they, like the mica and interstitial quartz, are secondary constituents. 



The garnets usually do not reach a magnitude of more than 2'"" 1 in diameter, 

 while their average is much smaller than this. In their growth they have 

 shown remarkable power in excluding or absorbing other minerals. Biotite 

 is generally not included at all in the garnets of the coarser, although quite 

 often included in those of the finer grained schists and slates. Quartz is 

 included to a greater or less extent, and in some cases quite largely. The 

 frequent absence in the garnets of the black ferriferous material which is 

 plentiful in the remainder of the section suggests that as the garnet has 

 grown it has absorbed much of that material. In the cases in which the 

 amount of ferrite or pyrite is very great the garnet has not been able to 

 wholly exclude or absorb them. In many cases the inclusions within the 

 garnets are more abundant near their centers than in their exterior portions. 

 This may mean that during the first part of their growth only was the fer- 

 rite being abundantly deposited. Often also the inclusions are arranged in 



1 reotogy of the Black Hills of Dakota, p. 70. 

 XXX — Bui.r,. (teoi.. Soc. Am., Vol. 1, 1889. 



