1895. | NEW YORK ACADEMY OF SCIENCES. 203 
is at times intergrown in alternate concentric layers with the 
chlorite of the vesicles, as is shown in Fig. B. 
Basie ash rocks occur abundantly and can sometimes be seen 
to be made up of diabase fragments. But most of them are too 
far altered to be certainly identified. 
PORPHYRITES. 
Under the group may be placed a considerable number of out- 
crops of strongly porphyritic basic effusives, mostly of purplish 
black color and not certainly known to grade into the diabases. 
They form heavy beds, apparently thick surface flows, very 
vesicular for the most part and containing phenocrysts of 
plagioclase which in one case, a porphyrite from south of Golden 
Grove, are remarkably fresh and glassy. 
Under the microscope they show a somewhat ophitic fine- 
grained groundmass composed of feldspar rods, minute grains 
of augite and magnetite in which are scattered the feldspar 
phenocrysts, those in the rock from Golden Grove being of the 
microtine habit, water clear, repeatedly twinned and somewhat 
corroded. No augite phenocrysts were seen. In a porphyrite 
from near Henry Lake the vesicles are filled with a zeolite, de- 
termined as thompsonite on its micro-characters. 
METAMORPHISM. 
The alteration as seen in these Coldbrook and Coastal rocks 
is very varied in amount. Devitrification is universal; in no 
instance has any trace of remaining glass been identified. The 
resultant microcrystalline mass varies in texture from a micro- 
felsite, in which the minerals are scarcely distinguishable to a 
moderately fine microgranite, in which are local accumulations 
of coarser grains. Shearing occurs sometimes in the porphyries, 
quite generally in the ash-rocks. In the Coastal especially, it is 
usually so much developed as to produce a schist. The last 
named alteration is accompanied by the production of great 
amounts of epidote; this mineral occurs less abundantly in the 
massive flows. Uralitic hornblende is a frequent alteration pro- 
duct of the diabase, retaining usually the form of the augite 
which it replaces. In no observed case is the change carried 
so far, either in the acid or basic rocks, as to produce a strongly 
schistose or subgneissic structure. It is not intended to assert 
that such rocks may not occur locally, but only that they are 
evidently not usual. 
