50 The American Geologist. January, 1892 
tions from the same magma. These basic masses are now in part 
much altered, but they may be classed with the d/orites. gabbros 
and peridotites. This gray granite formation is shewn to cover 
a much more extensive area than the schists, and it is held to 
constitute a geological unit and to be younger than the schists 
with which it is in contact. 
(2.) A reddish granite with garnets The feldspar is chiefly 
microcline, which is often in part intergrown with quartz in the 
manner of pegmatite. The mica is chiefly and sometimes exclu- 
sively muscovite. The coarser varieties of this muscovite gran- 
ite pass into pegmatite. This red granite is younger than the 
gray granite. The red granite also shews very commonly a 
parallel structure, but this cannot be ascribed so definitely to. 
pressure effects as in the case of the older granite, but it is rather 
to be referred to some primary condition. Associated with these 
formations there is in the middle portion of the region examined 
a long belt, varying in width from two to seven kilometers, 
composed of fine grained rocks rich in hornblende, of which the 
most typical variety possesses a well marked porphyritic structure, 
and is known as wrulitporphyrite. There are some minor varieties 
referred to, as melaphyre, plagioclase porphyrite, and amygdaloid, 
which are genetically allied to the typical uralitporphyrite. Out- 
side of this belt there are other occurrences, but of limited extent. 
Notwithstanding the great alteration which these rocks have under- 
gone, it is possible to recognize their original characters. They 
constitute a series which accords perfectly with some of the 
younger volcanic rocks. The occurrence of vesicles, of rocks 
which were originally glassy, and of tuffs and other volcanic 
ejectamenta is proof of the fact that for the development of these 
great eruptive formations there must have been a true voleanic 
activity. There was first an eruption of a comparatively acid 
magma, which solidified as a highly feldspathic andesite. There 
was at the same time, however, also a basic magma produced, 
from which arose melaphyre and plagioclase porphyrite. The 
cause of the metamorphism of these rocks is sought for in the 
strong mountain folding which occurred in pre-Cambrian time. 
By this agency the eruptive formations buried deep in the crust 
were folded together and altered apparently under the influence 
of solution. The relative age of these various geological forma- 
tions is determined by the following facts: The wralite porphyrite 
