214 TRANSACTIONS OF THE [APR. 15, 
netite in unusually large grains with a tendency towards skele- 
tal forms. The augite is quite strongly colored, and there is 
a not unimportant amount of free quartz (See Pl. XV., Fig. 2). 
The microscopic description is as follows : 
Augite nearly or quite as abundandant as the plagioclase, 
violet-brown, slightly but distinctly pleochroic, mostly moulding 
the feldspar, occasionally idiomorphice. 
Plagioclase in large, rather short lath-shaped crystals, idio- 
morphic with respect to the augite, water-clear, coarsely twinned. 
Extinctions are about those of labradorite. 
Magnetite is in unusually large grains, mostly of the peculiar 
skeletal forms which are sometimes referred to titaniferous iron ;* 
idiomorphic with respect to the feldspar and most, if not all of 
the augite, but with edges and angles frequently rounded. 
Apatite in long slender needles varying from .05 mm. in di- 
ameter down to indeterminable minuteness; included in all other 
constituents, though rarely in the magnetite. 
Quartz in allotriomorphic grains, moulding the feldspar 
and augite, forms a not unimportant constituent of the rock. 
It would seem to be original, judging from the unusual freshness 
of the rock, the size and uniformity of the quartz-grains, and 
the presence of apatite needles in them as abundantly as in the 
feldspar. 
An analysis of this diabase gave the following results : 
Si O, 46.61 
AEF, 0.55 
Al, O, 15.34 
Fe, O, 8.40 
Fe O 8.14 
Mn O 0.39 
Ca O : 9.27 
MgO 5.27 
Na, O 3.04 
K,0O 1.41 
Loss on ignition 1.41 
99.83 
The presence of free quartz in so basic a rock is unusual 
The large size of the magnetite crystals, however, probably in- 
dicates that the early stages of cooling were slow, giving time 
for the basic constituents to crystallize out’ more completely 
than usual, leaving at the end a more acid residue, which was 
able to slightly corrode the first formed crystais of magnetite, 
and produced some free quartz on crystallization. The same 
phenomenon is seen in the diorite-porphyrite. 
* The analysis, however, fails to show any considerable amount of titanic acid. 
