OF PRE-CAMBRIAN ROCKS 209 
either by appearance or by chemical analysis. The discovery of 
their genetic relations affords the key to the solution of some of the 
most important problems presented by this and other similar areas. 
The relative abundance of the limestones and their associated 
gneisses and amphibolites varies from place to place. In some 
parts of the area there are enormous developments of nearly pure 
limestones. Elsewhere these are very impure, while in some places 
there are great thicknesses of amphibolites and gneisses entirely 
free from limestones. 
The limestones with their interstratified and bedded amphibolites 
are usually steeply inclined, although in certain parts of the area they 
keep a nearly horizontal attitude. 
The thickness of the series cannot be determined, but it is very 
great. It is one of the thickest pre-Cambrian series in North America. 
Along its whole northern border this sedimentary series is torn 
to pieces by an enormous volume of gneissic granite of igneous origin 
which rises from beneath it, and which along its margin also wells up 
through it in the form of great intrusive bathyliths. This is filled 
with included fragments of the invaded series, limestones altered 
to amphibolites, fragments torn from interstratified bands of amphi- 
bolite, or of sedimentary gneiss, being scattered through it every- 
where and being sometimes found in a more or less digested condition. 
The base of the series has nowhere been found. Wherever its 
lower surface can be seen, it rests on great masses of the gneissic 
granite, which are intrusive into it and which, as has been mentioned, 
rise through it in the form of bathyliths. 
In addition to the granite intrusions, around the border of which 
in many places there are occurrences of nepheline syenite which afford 
one of the most interesting studies in the area, there are also several 
larger and a number of smaller intrusions of a basic gabbro and 
gabbro-diorite. These often show a wide range of magmatic dif- 
ferentiation. 
There are also a few smaller intrusions of anorthosite in the 
western part of the area, but these are distinctly subordinate. 
A few small intrusions of typical granite also occur, one of which 
in the township of Lake, forms the center of a development of acidic 
lavas and apparently represents the plutonic plug of an ancient volcano. 
