184 ROYAL SOCIETY OF CANADA 
as the surrounding rock, and differ only in the relatively greater abund- 
ance of the bisilicates present—indeed felspar and quartz are only sparingly 
represented, if at all. As the basic constituents are the first to cool and 
to assume a crystalline condition, these segregations mark the first formed 
nuclei in the slowly cooling magma. The result of pressure on a rock, 
characterized by the presence of these masses would result in their being 
squeezed or drawn out into more or less lenticular areas, assuming an 
approximately parallel position to one another in a direction at right 
angles to the pressure. 
Again, many of the dark bands present in these gneisses are 
seen to have had their origin as dykes which have been intruded in 
a direction corresponding to the foliation, as offering the least resist- 
ance. The origin of many of these dykes of hornblendic schist may very 
often be clearly made out in the field, as they can be frequently traced 
along their strike into areas or residual cores of the unaltered massive 
diorites or diabases which for some reason have escaped the pressure or 
deformation to which the surrounding rocks have been subjected. In 
the case of the more massive of these gneisses where the foliation is pro- 
duced by the approximately parallel position of the mineral constituents, 
their field relations, as well as their microscopical characters and chemical 
composition, reveal their true nature as irruptive granites which acquired 
a foliation as a result of pressure. In fact at the present time petro- 
graphers have abandoned the use of the term “ gneiss” as applicable to 
any definite type of rock, and are inclined to make use of it ina structural 
sense only or as a term of convenience when nothing very positive is 
known in regard to the constituents of the rock. 
It has been conceded that many of the plutonic rocks, such as gran- 
ite, diorite, etc., may, and, in fact, often do exhibit a tendency to a more 
or less parallel disposition of their component minerals so that it has now 
become customary where a rock has been subjected to examination, to 
speak of it as a gabbro-gneiss, diorite-gneiss, etc., in this way at once 
indicating the composition and texture of the rock mass. 
Recent work in Archean geology has likewise shown that a great 
deal of this gneiss has not only had an irruptive origin but was in a 
molten ora plastic condition at a time subsequent to the hardening of cer- 
tain distinctly clastic rocks with which it came in contact. At what depth 
below the surface these rocks must have solidified to produce their perfect 
crystalline condition is a matter of conjecture. Moreover, no certain evi- 
dence has been produced of the existence of any surface volcanic rock with 
which they might most reasonably have been expected to be associated, 
although subsequent denudation and erosion may have removed all traces 
of such mantle or covering. Although in many instances this first formed 
floor has been shown by its contact with the Huronian clastics to have 
been in a very unstable condition, still in a great many cases its present 
