Cushing — Potsdam Boundary. 9 



present structure being predominantly cataclastic, though this structure is 

 often masked by a greater or less amount of subsequent re-crystallization. 

 They vary from well foliated rocks to those in which all trace of this structure 

 has well nigh disappeared. In texture they vary from very finely granular 

 rocks to very coarse varieties. They abound in quartz and pegmatite veins. 

 According to their mineralogic composition they may be roughly classified 

 in three groups. 



(a) Quite acid gneisses, poorly foliated, commonly of red color, mostly 

 poor in content of f erro-magnesian silicates, and with the mineralogy and com- 

 position of granites. They consist essentially of microperthitic orthoclase and 

 quartz, with magnetite always present, and with an acid plagioclase, microcline, 

 hornblende, biotite, apatite, zircon, and rarely garnet as accessory minerals. 

 A strongly absorptive green-brown hornblende is the usual dark silicate and 

 becomes a prominent constituent in portions of the gneiss. Biotite is an 

 exceptional mineral in these gneisses and even when present is always subordi- 

 nate to the hornblende, biotite gneisses being of extreme rarity in the rocks of 

 this group in the northern Adirondacks at least. 



(!>) Gneisses whose main difference from those just described consists in 

 the predominance of microcline among the feldspars. In the field they are 

 often undistinguishable from the other gneisses. At other times owing to 

 their fineness of grain and their peculiar lilac grey or lilac brown shade on 

 fresh fracture, they appear quite distinct. They are so intimately associated 

 with, and pass so gradually into the other type that the wisdom of attempting 

 to distinguish them is by no means beyond question. They commonly occur 

 in proximity to, and may belong with the Grenville series. 



(6) Gneisses composed essentially of orthoclase, acid plagioclase and 

 augite, with accessory titanite, hornblende, apatite, magnetite and ilmenite, 

 quartz, garnet and biotite, in order of prominence. In composition they grade 

 from augite-syenites into gabbroic rocks, or from hornblende-syenites into 

 dioritic rocks according to the relative predominance of orthoclase or plagio- 

 clase. The more basic varieties, however, are more acid than the normal gabbros 

 of the region, their feldspars belonging to the oligoclase-andesme series, seldom 

 if ever becoming as basic as labradorite. The augites in these rocks are very 

 variable in character, ranging from a light-green, non-pleochroic diopside to 

 pleochroic varieties resembling aegerine-augite. The titanite is of a deep 

 orange color and is so constant and characteristic as to almost attain the dig- 

 nity of an essential constituent. Hornblende varies from complete absence to 



