278 The American Geologist. November, i899 
Whether, however, if traced out, they would be found to form 
rouo^hly parallel bands, or, as seems not unlikely, are connect- 
cd in circular form, is at present impossible to say. 
The oldest rock in this Archean region is a dark somewhat 
basic gneiss, with which is frequently associated crystalline 
limestone. This gneiss is cut by the igneous series which con- 
tains the corundum. These igneous rocks are themselves cut 
by quartziferous pegmatite and trap dikes. 
The corundum-bearing series in the main belt and in the 
smaller one, which lies to the southwest, consists essentially 
of syenyte-pegmatite and mica, hornblende and nepheline sye- 
nyte. Nepheline, it may be added, is found in some of the peg- 
matite. These syenytes vary greatly in size of grain and min- 
eralogical composition in dififerent parts of the field. In some 
cases, the syenyte masses appear to merge gradually into gran- 
ite, but corundum is absent in this rock. In other cases, well 
defined dikes of corundum-bearing syenyte occur, which have 
a width of an inch. It may be stated that in some parts of the 
main belt the syenyte passes into rocks which are basic in 
character and appear to have the composition of gabbros or 
anorthosytes. These latter rocks are at times highly meta- 
morphosed and have been changed into scapolite-bearing va- 
rieties. 
In the third belt, which has been referred to as lying to the 
southeast of the main belt, the corundum occurs in outcrops 
of basic anorthosyte, which were found across country for 
several miles. At the southwest end of the band, however 
so far as traced, the mineral again occurs in syenyte-pegmatite 
similar to that of the main 1)elt. We then have the mineral oc- 
curring in the region in rocks which range from those com- 
posed essentially of basic plagioclase to those whose chief con- 
stituent is alkali feldspar and nepheline. 
The anorthosyte band, in places, has a breadth of nearly 
one mile. So far as observed, this corundum-bearing rock 
does not occur in narrow dikes as do the syenytes, but in what 
may more properly be called masses. 
As corundum-bearing anorthosyte, so far as the writer i,s 
aware, has, up to the present, been found only in Russia and 
India, it seemed well to give an account of the composition of 
the Ontario rock in this paper. 
