38 
granules injected with epidote and the usual ferruginous decom- 
position products. In this ground-mass are numerous phen- 
ocrysts of feldspar and an occasional nearly colorless augite. 
There are numerous granular areas of secondary epidote ; but 
no trace of olivine or its decomposition products; and I am 
inclined to doubt its presence as an original constituent. Rec- 
ognizing the uncertainty attending the determination of such 
rocks from a single section, I can only say that I am inclined 
to believe the rock more nearly related to the augite-porphyrite: 
or olivine-free diabases than to the melaphyrs. It is classed 
here with the melaphyrs simply because the true nature of the 
rock is not yet free from doubt, and that arrangement best 
accords with its field relations as worked out by Professor 
Crosby. The analysis of a single specimen gave 54.47 per 
cent. of silica. 
The Porphyrite Dikes. 
The porphyrite dikes, so far as observed, are, lithologically, 
essentially similar to the great flow of porphyrite on Black 
Rock, rendering a detailed description unnecessary. 
The Diabase Dikes. 
These dikes are, as a rule, exceedingly compact, tough and 
hard, of a dark gray, nearly black or more frequently greenish 
color, and carry few if any original macroscopic constituents, 
although pyrite and secondary epidote are often developed in 
granules and aggregates of sufficient size to be detected by the 
unaided eye. To this secondary epidote and the abundant viri- 
dite is due in all cases the green color of the rocks. 
I find nothing whatever in the slides, with possibly a single 
exception, to indicate that the different dikes belong to distinct 
periods of eruption. Slides from dikes which plainly belong to 
the same system often show all the variations in texture, struct- 
ure and stages of decomposition to be observed in those which 
