778 A. P. COLEMAN 
between the plagioclase, and rarely a little micropegmatite. A 
small amount of pyrrhotite is found at two or three points along 
the band, but inclosed within the rock, not at one edge as in the main 
range. 
The most interesting feature of this gabbro band is the great 
white masses of a much more acid rock which are found irregularly 
along the top of the ridges east of Sudbury and south of Copper Cliff. 
Some of these masses are Ioo yards across and make a very striking 
contrast with the general green-gray gabbro. They are always 
inclosed in a margin of very coarse, dark-green hornblende, followed 
by a zone of mixed hornblende and white feldspar in anhedra several 
inches across. ‘This phase is succeeded by a zone of white binary 
granite showing coarse pegmatitic structure, inclosing a central area 
of almost pure quartz, which may be 50 feet wide. 
The hornblende often forms long prisms with a core of white feld- 
spar. The feldspars are oligoclase approaching albite, and orthoclase 
(or an unstriated plagioclase), and the quartz is glassy, like vein quartz. 
It is possible that these curious masses, so much richer in silica and 
alkalies than the rest of the rock, are segregations of the magma 
separated from the more basic parts somewhat like pegmatite dikes 
in granite; but they are more probably products of the digestion of 
large blocks of Huronian quartzite stoped from above. They occur 
always on the highest points of the gabbro hills. 
Very much smaller and less striking masses of a similar kind are 
found on the basic edge of the main nickel range near Murray mine, 
close to the bottom of the laccolithic sheet. This fact and the small 
amounts of pyrrhotite found in the gabbro band just described suggest 
a relationship between the two rocks. It may be that this gabbro 
originated in the nickel-bearing magma before segregation had 
advanced very far, long before the great sheet was injected into its 
present position. 
The three types of rock described above as probably products of 
differentiation of the main magma, basic norite, acid granite, and 
intermediate gabbro, were no doubt erupted at very different periods; 
the gabbro probably coming first, the basic lava flows next, and the 
granite partly before the ascent of the nickel eruptive and partly after 
it. None of these rocks occupy areas of more than a few square 
