MICROPEGMATITE IN PURCELL SILLS. 
29 
and no evidence of assimilation is present. The blocks may have 
been torn from the floor of the sill and risen to their present 
position. Such a phenomenon has been described by Campbell 
and Stenhouse . 1 Here a block of sandstone has been disrupted 
from the floor of a sill of teschenite and risen to the top of the 
sill and left trailers in its path. Thus underhand, as well as 
overhead, sloping is possible in a tabular intrusive body. 
Bailey , 2 in his paper on the “Eruptive and Sedimentary 
Rocks of Pigeon Point, Minnesota,” emphasizes assimilation in 
the following words: — 
“More direct evidence of the action of the gabbro on the quartzite ia found in 
the inclusion of the latter in the former. It will be remembered that the alteration 
of the fragments in the gabbro is in general similar to that of quartzitic fragments 
in the red rock. In the latter in certain cases the quartz fragments are surrounded 
by a rim of red material, which, under the microscope, presents all the appearances 
of the red rock, except in the presence of green flecks of chlorite. Quartzitic frag- 
ments in the gabbro are bordered by a rim exactly like the material in the red rock. 
At a point on the southern shore, in the eastern portion of the peninsula about i mile 
from the end of the point the rock cementing quartzite and slate fragments is similar 
to one of the rocks intermediate between the gabbro and the red rock, and whose 
formation is supposed to be due to the interfusion of these rocks. The origin of the 
cementing material of this breccia may be the direct solution of fragments in the 
gabbro. One of the fragments embedded in the intermediate rock is a large rhom- 
bohedral block of pink quartzite about 7 feet long and 4 feet wide. Surrounding 
this, between it and the including rock, is a bright border 2 £ or 3 inches wide. The 
red feldspathic material has a granophy ric structure in wh ich fan-like groups of feldspar 
and quartz extend perpendicularly from the bounding planes of the inclusion. Since 
the rim is probably the result of the fusion of portions of the quartzite by the surround- 
ing rock, and its structure and composition are identical with those of the red rock, 
it may fairly be concluded, in the absence of any evidence to the contrary that the 
red rock itself has been produced by the fusion of the quartzites by the gabbro — that 
it is a product of the action of the gabbro upon the slates and quartzites, melting 
the latter and thus producing a magma from which the red rock solidified.” 
In Bailey's concluding chapter on the origin of the red rock, 
he states that no positive determination can be made whether 
the gabbro is the cause of the red rock or the red rock is an 
original eruptive. Another solution to this problem might be 
given. If the intrusion is in the form of a sill, it may have been 
intruded as a magma of intermediate composition which later 
differentiated into gabbro and red rock. As the red rock 
contained most of the mineralizers, contact metamorphism 
would be greater in connexion with this phase than with the 
other and some assimilation of the enclosing quartzites would 
take place. This view is supported by the occurrence of heavy 
contact metamorphism only where the red rock has collected 
‘Campbell and Stenhouse, Trans, of Edin. Geol. Soc., vol. 9, 1908, p. 121. 
2 Bailey, W. S., U. S. G. S., Bull. 109, 1893. 
