CHARACTERISTICS OF AZOIC ROCKS. 139 



composition. There is, however, no reason to suspect any funda- 

 mental changes in the granitic or hornblendic rocks or schists. 

 But the potstones, both magnesian and aluminous, are probably 

 of later origin. The Rensselaerite has been observed under the 

 crystalline form of pyroxene, showmg that in part, at least, it has 

 been made out of pyroxene ; and the aluminous species exists 

 under the crystalline form of nepheline, giving unequivocal proof 

 that it has been made out of pre-existing nepheline crystals, like 

 the gieseckite of Greenland, which it resembles in aspect and com- 

 position. The rocks are probably, therefore, the result of the altera- 

 tion of different minerals or rocks after the first Azoic crystalli- 

 zation. If this be true, they may not be actually Azoic rocks : they 

 may belong to the same age with the metdmorphic rocks of New 

 England, or to some other period. By one interested in bringing 

 the events of geological history into their true chronological rela- 

 tions, — the real end in geological studies, — this will be regarded as 

 an important question. 



Other evidences of alteration since the original crystallization in 

 northern New York have been observed, — such as the rounded 

 quartz crystals of Gouverneur, and the soft spinels of St. Lawrence 

 co., called houghite. Even the serpentine of the same region may 

 come into this category. 



Minerals of the Azoic rocks. — Besides the constituent minerals mentioned, — viz., 

 quartz, feldspar of different species, hornblende, pyroxene, epidote, mica, talc, 

 garnet, — there are also the following common species : tourmaline, scapolite, 

 wollastonite, sphene, rutile, graphite, the mica called phlogopite (§ 56), apatite, 

 chondrodite, spinel, zircon, corundum, — each of which occurs at times in the 

 crystalline limestone or its vicinity. Mica is found in Grehville, Canada, in 

 plates between one and two feet square. In addition to these, there are a 

 number of rare ores of yttrium, cerium, and columbium among the Swedish 

 Azoic rocks. 



No gold has thus far been found in the Azoic. Andalusite, kyanite, and 

 staurotide are also among the common minerals of crystalline schists not 

 detected in the Azoic. 



Characteristics of the Azoic rocks. — 1. The Azoic rocks are 

 nearly all crystalline rocks. A few sandstones, slates, and conglo- 

 merates are the only exceptions ; and these are excessively hard 

 rocks. 



2. The crystalline rocks are remarkable for the small amount 

 of silica they contain (a fact noticed by T. S. Hunt). This is 

 seen in the absence of quartz from many of the rocks (the diorite, 

 Labrador rock, hyperite), and the abundance of feldspars, like 

 labradorite, that have a low proportion of silica. 



