1894. ] NEW YORK ACADEMY OF SCIENCES. 191 
change, since it has been observed only in those slides which 
show evidence of shearing and strain, and is abundant in such 
specimens as have a marked gneissic structure. What look 
like transitions between orthoclase and microcline are sometimes 
seen. 
Microperthite is not uncommon, sometimes forming a partial 
ring around plagioclase crystals at their contact with quartz 
(Fig. 2), more often scattered in small masses between the other 
constituents. In the former case it looks rather like a reaction 
rim, and may be produced in a similar way. 
The specific gravity varies from 2.69 in the coarse porphyritic 
granite to 2.75 in the fine dark grey quartz-diorite. A partial 
analysis of a specimen as nearly typical as could be obtained 
gave the following results : 
SiO . 65.8 
ATES Oc oes 
HeO), Ke,'O-, cr 
Ca O 3.5 
Mg O : : 0.9 
Ke: O77 Nag O- 6.6 
Loss on ignition not determined. 
The high percentage of iron is rather remarkable. In this 
respect it agrees well with the analysis published by Vom Rath 
of the original Tonalite from the Alps. 
VartatTion. The most striking change in the character of the 
rock is the greater or less abundance of orthoclase phenocrysts, 
which are in largest amount in the widest intrusions, while in 
the narrower ones they become smaller and fewer and finally 
disappear, being only partly replaced by an increase in the 
amount of orthoclase in the ground-mass. As the orthoclase 
decreases, the rock becomes finer in grain and darker in color, 
hornblende coming in in larger proportion and biotite almost 
disappearing. The amount of quartz does not seem to vary to 
any extent. Here and there are seen small dark patches, com- 
posed of the same constituents as the surrounding rock, but 
with hornblende in excess. These occasionally take the form of 
dikes, apparently due to extravasation into fissures in the half- 
solidified magma; the common rounded patches being more 
probably segregations at an earlier stage. True dikes, of quite 
different character and appearance, are abundant near the edges 
of the intrusion; most of these are porphyritic diabase. 
RecionAL METAMORPHISM. This is confined to the edges of 
the quartz-diorite and the vicinity of faults, the main mass being 
little affected. A well defined gneissic structure is seen in 
