THE IGNEOUS ROCKS. 509 



less cliloritized, in an altered groundniass filled with skeleton crystals of 

 magnetite. Apparently this groundmass was originally glassy. It now con- 

 sists of chlorite and certain indefinite, brownish -green, fibrous substances 

 in a light-colored matrix that polarizes feebly in places, like an altered 

 feldspar, and in other places acts like an isotropic substance. The second 

 rock has been entirely recrystallized. It now consists of large, fresh plagio- 

 clases and small areas of quartz mosaic in a groundmass cut through and 

 through by light-green amphibole crystals resembling actiuolite. The 

 interstitial substance between these is a mosaic of quartz and plagioclase. 

 There are among the dike rocks a few other porphyrites, but they are coarse- 

 grained, and differ from the nonporphyritic ])hases merely in that they 

 contain phenocrysts of plagioclase. 



The most schistose phases of the dike rocks include the chlorite-schists, 

 the talc-schists, and the kaolin-schists. 



In the chlorite-schists chlorite predominates over all other components. 

 The rock of the dike at the old Gilmore mine (Atlas Sheet XXXV), near 

 the north quarter post of sec. 26, T. 47 N., R. 26 W., for instance, is an 

 aggregate of bundles of a fibrous green chlorite, through which are 

 scattered laths of plagioclase. The latter can be seen only in polarized 

 light, since in natural light the green color of the chlorite obscures them. 

 A little limonite stains the chlorite here and there, and a few magnetites 

 are besprinkled through it. Other chloritic schists are very similar to this 

 one, though not many of them are so nearl}' devoid of components other 

 than chlorite. 



In the rock of two dikes, one at Humboldt and the other 835 steps W., 

 700 steps N., of the SE. corner of sec. 24, T. 48 N., R. 31 W., are great 

 plates of chloritoid of the same character as that in the western knob 

 greenstones described on page 504. In the first dike (Specimen 14777) the 

 chloritoid is in beautiful fresh columnai- crystals, having an extinction 

 varying between 0° and 16° and twdnned parallel to their longer axes, 

 and in large plates with the cellular structure of secondary minerals, 

 which plates are probably basal sections of the columnar crystals. The 

 chloritoid, with fibrous chlorite, forms a network, in whose meshes are 

 flakes of colorless muscovite, little gi-ains of epidote, and grains of quartz. 



