"74 A. P. COLEMAN 
pletely digested, while later the eruptive became too viscid to allow 
blocks to sink, but worked on the overlying rock by heated vapors or 
solutions, causing the remarkable blending of the eruptive with the 
conglomerate. As the rock above became heated it would lose in 
specific gravity, until ultimately it might not have sufficient weight 
to sink into the magma, which was all the time growing cooler and 
heavier. 
Another factor which may be of importance is the gradual change 
in composition of the upper part of the magma as it absorbed materials 
richer in silica, whether by digestion of sunken blocks stoped from 
above, or by direct solution of overlying rocks. It is well known 
that the more siliceous lavas are much less fluid when near their 
melting-point than basic lavas, so that the upper part of the sheet 
may be supposed to have become less and less fluid until blocks could 
no longer sink through the viscid mass. 
It may be that all three processes took part in the differentiation of 
the eruptive, and that magmatic segregation and the rising of the 
more acid portions proceeded along with the stoping of blocks and the 
direct absorption of the overlying rocks. 
It is worthy of remark that the micropegmatitic structure is found 
only in connection with the nickel eruptive; the other acid eruptives 
of the region are quite free from it. Can it be that micropegmatite 
is a structure specially belonging to rocks where basic magmas have 
stoped down and digested or otherwise absorbed more acid rocks ? 
Dr. Daly’s sills and the Pigeon Point rocks seem to support this as well 
as the Sudbury sheet. 
EARLIER AND LATER DERIVATIVES OF THE MAGMA 
Basic norite-—Though the nickel-bearing eruptive is by far the 
most important mass of igneous rock in the region, there are several 
other eruptives that appear to have split off from the same magma 
at earlier or later times, and these may be more briefly described. 
They include an older norite which very frequently underlies the 
nickel-bearing norite, though of a very different type; and certain 
granites which are intimately associated with the laccolithic sheet. 
From point to point for several miles along the southern edge of 
the main range there is a complex mass of fine-grained norite and 
