58 Adams—Contribution to our knowledge of the Laurentian. 
Art. VII.—A further Contribution to our knowledge of the 
Laurentian ; by FRANK D. ADAms. With Plates I and II. 
THE chief of the great protaxes of the North American 
Continent, having an area of about two million square miles, 
lies as is well known for the most part in the Dominion of 
Canada, and in it Logan distinguished two great systems or 
series of rocks—the Huronian and the Laurentian—the former 
being a distinctly clastic series, while the latter showed no 
undoubted clastic structure but consisted chiefly of gneisses of 
various kinds. He also found associated with these Laurentian 
gneisses in certain parts of the protaxis an abundance of quartz- 
ite and erystalline limestone, as well as much anorthosite and 
eventually divided the Laurentian into two parts, viz: the 
Upper Laurentian—consisting of the anorthosites (supposed to 
be stratified rocks), with some of the gneiss and crystalline 
limestone, and the Lower Laurentian consisting of the Gren- 
ville series, distinguished by the presence of numerous bands 
of crystalline limestone, quartzite, etc., resting upon the Fun- 
damental Gneiss. 
These two divisions, the Upper and Lower, were supposed 
to be unconformable. 
In a paper which appeared two years ago in the Neues Jahr- 
buch fiir Mineralogie,* under the title “ Ueber das Norian 
oder Ober-Laurentian von Canada,” it was shown from a study 
of all the areas of the so-called Upper Laurentian, that the 
rocks which had been assigned to it do not constitute an inde- 
pendent series, but that the anorthosites occur as a series of 
great intrusions, while the associated gneisses and erystalline 
limestones are merely portions of the Grenville Series or Fun- 
damental Gneiss as the case may be. 
In a paper which was recently presented to the Geological 
Society of America, moreover, Ells states it as his belief from a 
reéxamination of Logan’s typical area that the crystalline lime- 
stones of the Grenville Series are not concentrated at some 
four widely separated horizons as Logan supposed, but come in 
gradually, as interstratified bands, attaining their principal 
development at the summit of the series. 
It appears, therefore, that what has been called the Lauren- 
tian in Canada consists of an underlying series of gneisses and 
granites called the Fundamental Gneiss, much of which may 
be and probably is of igneous origin, and an overlying series of 
gneisses often differing from these in petrographical character, 
associated with crystalline limestones and quartzites, and known 
as the Grenville Series. 
* Beilage Band, viii, 1893. 
