[ELLS & BARLOW] PROPOSED OTTAWA CANAL 183 
fibrous radiating crystallizations of a dark green hornblende. It occurs 
in rather small quantities, and, like the Talon Lake limestone, seems to 
have become incorporated in the gneiss during its irruption, On Iron 
Island, in Lake Nipissing, small patches of a dark coloured crystalline 
rock, containing a large proportion of calcite, has been sometimes described 
as a crystalline limestone, but an examination under the microscope reveals 
its irruptive character, the large quantity of calcite present resulting 
from the decomposition of the plagioclase originally present. | 
It is not possible here to go into the various theories in regard 
to these ancient crystalline rocks, but it may be well to mention 
certain facts in this connection. It may be safely stated that the old 
belief that the whole of these gneissic rocks represent ordinary aqueous 
sediments which have undergone such extreme metamorphism as to 
mask their original character, is now entertained by few geologists. The 
“basement complex,” as it has frequently been called, has been shown to 
be composed of a great variety of rock types whose sole resemblance con- 
sists in the more or less parallel disposition of their component minerals. 
The excellent work of Professor Lehmann on the gneisses of Saxony gave 
a fresh stimulus to the study of these old rocks, and the rapid advances 
lately made in the science of petrography have caused geologists to take a 
fresh interest in them. 
The work of Dr. A. C. Lawson, for the Geological Survey of Canada, 
in the Lake of the Woods and Rainy Lake districts, already referred to, 
accompanied by an extensive microscopical examination of the rocks of 
these districts, as well as the detailed petrographical studies of the late 
Professor G. H. Williams in the Menominee and Marquette region of 
Michigan, defined clearly the fact that the foliation or lamination of these 
crystalline rocks does not necessarily imply an original bedded character, 
but in most instances has been the result of the enormous mechanical 
stresses to which these rock masses have been subjected. The studies of 
both these observers in the field, as well as in the laboratory, led them to 
the belief that many of the crystalline rocks, which had formerly been 
regarded as altered sedimentary strata, were in reality of irruptive origin, 
and distinctly referable to the more commonly massive plutonic rocks, 
The foliation noticed in these gneisses is produced either by (1) the 
alternation of light and dark bands, or (2) by the more or less parallel dis- 
tribution of the component minerals. In many of the plutonic or deep- 
seated rocks, particularly the granites and similar allied types, a marked 
tendency is noticed in the bisilicates to aggregate themselves in certain 
spots or patches, leaving the rest of the rock mass comparatively free 
from these constituents. The dark spots (Ausscheidungen) so conspicuous 
in many granites are perhaps the most familiar examples of this ‘“ mag- 
matic differentiation,” as it has been called. A microscopical examination 
of these “spots” shows that they possess the same mineral constituents 
