PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 723 



Greenwacke. ] 



Mic. The most of the rock is constituted of a varying granulitic association, 

 supposed to be in part of quartz and feldspar, but so fine that it is difficult in some 

 cases to prove it. This variation consists in the coarseness and the fineness of this 

 substance, and in -the consequent degree of translucency which it presents. Some- 

 times this variation is caused principally by the presence in varying amount of a' 

 greenish coloring matter. This matter is composed of globular epidote, of adinolitc 

 fibres and of chlorite, and it is evenly distributed throughout any individual space, 

 differing from space to space, but it is always accumulated about the borders of the 

 spaces in distinctly greater amount, so as to outline the spaces and separate them one 

 from the other. These spaces are evidently those formerly occupied by volcanic 

 glass or by fragments of minerals such as feldspar and perhaps by some ferro-magne- 

 sian mineral. Their shapes can be seen best by using converging light and by 

 lowering the condenser. Sometimes there is a remnant of the old feldspar still 

 visible in the central part of a space, which indicates the nature of the original 

 grain. If this be the nature of these variations, they may have been (in some cases 

 foregoing) mistaken for pebbles of devitrified glass, but in general they do not 

 appear to be derived from feldspars. Again, if this be their nature, or whatever 

 their nature, it is to be explained how it happened that, scattered through the slide, 

 there are still preserved crystal fragments both of hornblende and of feldspar that 

 show no such change, some of the former of which have slight 

 increments of later date. These are accompanied by quartz in 

 rounded and angular form, evidently fragmental in its present 

 place. Such an association as shown by figure 45 rather indicates 

 that this was a pebbly rock. Here, in a roundish space, is an 

 altered angular area, the two having different coarseness and different trans- 

 parency. The crystal was ophitically embraced in the matter surrounding it, and 

 both have suffered a micro-granulation or alteration of the same kind. Whether 

 this took place in the present rock or in some rock from which both were derived is 

 an important question. The existence of unchanged crystals in this rock points to 

 some earlier state in which such change as the above was effected, and that by some 

 agency the two were brought in a pebbly state, along with fresh crystal fragments, 

 and were contributed to the formation of the present rock. This rock is illustrated 

 by plate III, figure 1. One section. 



Age. Archean (Upper Keewatin). 



Remark. Material like the pebbly forms seen in this rock, but usually more 

 broken and lost by friction and decay, is apparently quite commonly distributed 

 amongst the clastic greenstones. N. H. w. 



