1894. | NEW YORK ACADEMY OF SCIENCES. 197 
Cambrian and later rocks, may be taken as some evidence as to 
the age of the former. The granite, as well as the Laurentian 
series, is traversed by innumerable dykes, while in the Palaeo- 
zoic rocks they are very rare, the only ones I have seen being 
in the Lower Carboniferous shales near Poverty Hall Point, and 
these are of a different character from the ordinary. In the 
Huronian there are a good many dykes in places, but the dia- 
base is mostly interbedded. If, then, the diabase dykes approxi- 
mate the Huronian in age, as seems not unlikely, the granite 
must be earlier than this, and belong to the interval between 
the Laurentian and Huronian. Again, the pre-Carboniferous 
rocks, though much folded and considerably faulted, are hardly 
so much changed as to suggest an igneous intrusion of such 
prevalence as this, being connected with their metamorphism. 
Altogether it seems more probable that the granite is pre-Cam- 
brian, and very likely pre-Huronian, for the great unconformity 
here is between the Laurentian and Huronian, not between the 
Huronian and Cambrian. 
Il. GABBRO. 
Within the area as yet studied, the gabbroitic rocks are con- 
fined to two small knobs at Indiantown, where they are mostly 
wellexposed. Another larger body occurs north of Dolin’s Lake, 
some seven or eight miles northeast of the city, and has been 
described in the Canadian Survey Reports,* as a norite of basic, 
but very variable character. About half-way between these 
two localities, but outside the accompanying map, is a little ex- 
posure of an altered gabbro, surrounded and almost buried by 
drift. The Indiantown gabbro was described, but not recog- 
nized in the survey reports, the unaltered rock being not very 
accessible, and was called ‘ diorite” and “ feldspathic diorite.” 
The knob fronting on the river, which gives the best expos- 
ures, varies from anorthosite at the southern end to peridotite 
at the other. The anorthosite is moderately coarse, the grains 
averaging 4 to 4 inch diameter, seldom much more. The amount 
of dark silicate present is sometimes almost nil; where it is a 
little more abundant, the rock sometimes appears strongly por- 
phyritic, the feldspar being in well-developed thick tabular crys- 
tals in a matrix now composed of pale green actinolite. To- 
wards the northern part of the exposure, more dark silicate 
comes in, mostly aggregated in irregular masses, which, decay- 
ing much more easily than the feldspar, give the weathered sur- 
face a characteristic pitted appearance, like that noted by Prof. 
* Can. Gol. Sur., 1871, p. 41. In Amer. Nat., Nov., 1885, Prof. Adams, having 
examined thin sections ot this rock, states that it is an olivine- -gabbro, 
