1895.] Petrography. 995 
the abundance in them of microperthitic intergrowths of orthoclase and 
plagioclase. From the relations of the plagioclase to the orthoclase 
and to the surrounding minerals there can be no doubt that it is of 
secondary origin. It fills cracks between quartz and orthoclase, and 
from these areas it sends long stringers into the orthoclase along its 
cleavage cracks and into its fracture lines, without suffering the least 
interruption in its continuity. The gneiss in its structure is sometimes 
granular and sometimes granulitic, and in the appearance of its con- 
stituents it shows plainly that it isa dynamo-metamorphosed rock. The 
dark bands occurring with the predominating light colored ones zon- 
sist, as a rule, of the same minerals as the latter, but one band noted is 
composed of monoclinic pyroxene and hornblende in addition to the 
feldspars. The normal granites of the region differs in composition from 
the gneiss in the absence from them of hornblende, except in certain 
basic segregations. The granite, like the gneiss, has suffered the effects 
of pressure, but to a more limited extent. Among the limestones 
associated with these rocks are phases containing much colorless py- 
roxene, tremolite and scapolite. Near the base of the limestone series 
the pyroxene-scapolite rocks are foliated, and are apparently interstrati- 
fied with unaltered beds. They consist of feldspar, quartz, pyroxene, 
mica, sphene, apatite, graphite, pyrrhotite and pyrite, or of these com- 
ponents, with the feldspars replaced by secondary scapolite. 
Diorites and Gabbro at St. John, N. B.—Among the in- 
trusive rocks cutting the Laurentian near St. John. N. B., Matthew’ 
finds a granite-diorite and a gabbro. The diorite is coarse grained and 
porphyritic in its larger masses, and fine grained and granular in its 
smaller bands. Quartz, plagioclase, orthoclase, hornblende, biotite and 
the usual accessory constituents compose the rock, while epidote and 
microcline-microperthite are present in it as alteration products of 
plagioclase and orthoclase. The microperthite is also noted as forming 
a rim between plagioclase and quartz. As the rock becomes finer 
grained orthoclase and biotite diminish in quantity. Although the 
contacts of the diorite with the surrrounding rocks are usually faulted, 
it can be clearly seen that the latter have been altered by the intrusive. 
On the contact with a gabbro, this latter rock has been changed to a 
granular aggregate of hornblende and plagioclase. The diorite, on the 
other hand, is very fine grained, and is composed of an allotriomorphic 
mixture of plagioclase, quartz, orthoclase and a few small shreds and 
grains of hornblende and biotite. Limestone in contact with the 
3 Trans. N. Y. Acad. Sci., xiii, p. 185. 
