4.SS 



Report of the State Geologist. 



ance. They form wide areas between belts of sedimentary rocks, but 

 themselves, as above stated, showing no trace of sedimentary origin. On the 

 contrary, they show the uniformity of character that might be looked for in 

 plutonic r<x ks, together with the gradual variation so common in rocks of this 

 class. Such parallel structures as appear in the gneisses have been shown to 

 have nothing in common with the bedding of a sedimentary series, while they 

 are precisely such structures as appear in plutonic rocks as a result of 

 pressure, or of flow before solidification. 



Sections of the widespread, fine, light-colored gneiss show it to consist of 

 orthoclase, plagioclaee, and quartz, with mica, hornblende or pyroxene. Either 

 feldspar may predominate, and microperthite is nearly always present. As a 

 rule, the ferro-magnesian minerals are in small amounts. With the exception of 

 the quartz, the constituents are in grains, which often appear to have resulted 

 from the granulation of larger grains, that is, the structure is cataclastic and 

 the original rock must have been a coarse, holocrystalline aggregate, such as is 

 typically afforded in -the class of plutonics. Such a rock under pressure 

 would yield the gneisses, and at many points the latter pass by insensible 

 gradation into such a rock. That they have been derived from it by pressure 

 is clearly indicated. In some cases this massive core is a coarse, evenly 

 granular aggregate, in others it is more or less distinctly porphyritic. The 

 latter is perhaps the most common type, forming considerable and widely 

 distributed areas. The case is analogous to that of certain gneisses of 

 Canada, in w hich, as shown by Adams,* the cataclastic structure is usually 

 accompanied by other indications of an igneous origin ; while, in the same 

 legion, gneisses lacking this structure are thought to be derived from 

 sediments. The fact that a gneiss is formed by crushing of a coarse 

 holocrystalline aggregate does not prove its igneous origin, as this aggregate 

 may have been formed by crystallization of sediment. But in the absence 

 of distinct proof of the latter supposition, the probabilities favor the former; 

 and if all other kinds of evidence lead in the same direction, the cataclastic 

 structure may be regarded as an important indication of igneous origin. 



The role of the quartz in the gneisses requires some special consideration, 

 being, as above indicated, somewhat exceptional. Quite often the mineral 

 show s the effect of crushing less than do the other constituents. While they 

 are in small rounded grains, the quartz is often in large masses or long spindles. 

 A- the quartz could hardly flow while the feldspar fractured, the conclusion 

 is obvious, and seems to be well grounded, that, in the case of the quartz, there 



* American Journal of Science ; (III) L., pp. 5H CO (11). 



