Relations of the lgenous Rocks — Crosby. 45. 
Quartz Porphyry — This upper or peripheral member 
of the contact zone has been designated by Dr. Bascom the 
rhyolitic facies of the granite or more succinctly rhyolite, 
and more explicitly porphyritic aporhyolite ; and for this 
usage the petrographic characters undoubtedly afford 
ample warrant. But in order the more sharply to distin- 
guish this essentially plutonic type from the much younger 
and very dissimilar intrusive and effusive rhyolites, it is 
proposed to employ here the good descriptive term quartz 
porphyry. The rock in question is in every instance a true 
quartz porphyry, with conspicuous phenocrysts of both 
quartz and feldspar; and, as befits its plutonic origin, it is 
of remarkably uniform character, matching the granites in 
this respect; while the clastic, fluidal and spherulitic struc- 
tures so characteristic of the newer rhyolites are conspic- 
uous by their absence. Such variation as the quartz por- 
phyry shows is due chiefly to its gradation downward into 
the fine granite ; and, as Dr. Bascom has noted, its texture, 
though aphanitic, allies it with the microgranitic phase of 
the fine granite, and unlike the younger rhyolites it is rarely 
truly crypto.crystalline. 
In its distribution the quartz porphyry tends to form 
a V-shaped zone concentric with the fine granite, and sepa- 
rating the underlying fine granite from the overlying 
effusive rhyolite or felsite. The lower border of the quartz 
porphyry is rendered rather vague and indefinite at most 
points by its blending contact with the fine granite. The 
upper border, on the other hand, where the quartz por- 
phyry meets the effusive rhyolites or felsites is, in the na- 
ture of the case, sufficiently definite but highly irregular, 
since we have here a true erosion unconformity, and two 
formations, although of closely similar composition and 
probably derived from the same original magma, are strong- 
ly contrasted in structure and widely separated in geolo- 
gical time. 
Summary and Comparison — Neglecting unimportant 
occurrences of diorite and aplite, which may be described, 
respectively, as relatively basic and relatively acid phases 
or segregations of the normal granite, and hence as pro- 
ducts of a chemical differentiation of the main bodv or 
