68 Adams— Contribution to our knowledge of the Laurentian. 
fore a rock, which so far as its composition is concerned, might 
be either an altered sediment or an altered igneous rock, and it 
is impossible therefore to draw from its chemical composition — 
any definite conclusions as to its origin. | 
Many other gneisses including some in the Grenville Series, 
especially those of class II, have undoubtedly the composition 
of igneousrocks. Asanexample of these, Logan’s typical Funda- 
mental gneiss from Trembling Mountain was selected. An 
analysis of it is given under No. VIII. It forms almost the 
entire mass of Trembling Mountain, a long ridge rising on the 
east side of Trembling Lake to a height of 2500 feet and form- 
ing the highest point in the Laurentians in this part of Canada. 
It is a rather fine-grained gneiss, uniform in character and is 
under the microscope plainly seen to be a crushed or granulated 
hornblende granite. The analysis shows it to possess a chem- 
ical composition quite different from that of the other gneisses 
and slates just described. The silica is high but the alumina 
comparatively low. The alkalies are also high, the percentage 
being that which is commonly found in granite, while the lime 
preponderates largely over the magnesia. The analyses of the 
granite from the Carlingford District in Ireland by Haughton 
(LX) will serve to emphasize this identity. 
In the cases of those gneisses then, Nos. I, H, V and VII, 
whose stratigraphical relations and microscopical character sug- 
gest a sedimentary origin, the first three have the composition 
of slates, that is to say of clay; in the case of No. V of clay 
mixed with sand, while in the case of No. VII no definite con- 
clusion can be drawn. 
No. VIII on the other hand, which from its stratigraphical 
relations and microscopical character can be recognized as a 
crushed granite, has an entirely different chemical composition 
and one which is identical with a normal granite. 
With regard to the rocks of class III therefore we find that: 
(1) The presence of numerous and heavy beds of limestone 
and quartzite. (2) Their prevailing banded character, accom- 
panied by a very extensive recrystallization. (8) The frequent 
occurrence of graphite in all rocks of-the class, and (4) The 
fact that the gneisses of this class have in many cases at least 
the composition not of igneous rocks but of sands and muds— 
combined to make it extremely probable that we have, in the 
case of many of these rocks at least, extremely altered forms 
of very ancient sediments. 
The Fourth Class, composed of pyroxene gneisses and 
pyroxene granulites, comprises a set of rocks which are very 
abundant in this particular area and occur associated with both 
of the other classes of gneissic rocks. Lehmann* from his 
* Enstehung der altkrystallinischen Schiefergesteine, pp. 230-236. 
