112 
AMERICAN GEOLOGY. 
lands and the head of the valley of Champlain. This part of 
the belt, together with the more northerly part of it, between 
Montreal and Quebec, is upon a line of junction between two 
systems or formations, and the juncture or belt in proximity 
with it, is made up apparently of the thinnest masses of the 
systems, and hence is a line of weakness. If this position is 
true, then, and if it has been one of great tension, it explains 
the fact of the fracture and disturbance upon the line. 
Again, upon lake Superior several distinct parallel belts ot 
trap range in a northeast direction, taking, however, a curvili¬ 
near course. These traps consist of compact greenstone, 
amygdaloid, and basalt; they are also chloritic and ferruginous. 
The compact variety passes into granular semi-crystalline trap, 
in which feldspar is visible. The rock is regarded as a pro¬ 
duct formed by the fusion of labradorite and hornblende. The 
range extends from the extreme limits of Kewaunee point to 
Montreal river. Isle Royal is composed of materials similar to 
Kewaunee point. The geological investigations prove that the 
trap belongs to the oldest Silurian period, as it alternates with 
the Potsdam sandstone. Eruptions of trap took place while 
the sandstone was in the process of formation. This turns, it 
is true, on the correctness of the determinarion of the age of 
the magnesian limestone, which rests upon the sandstone. If 
this is equivalent to the calciferous and chazee limestones of 
New York, there can be no doubt respecting the age of the 
underlying sandstone. 
It will be observed that the trap is not intruded between the 
layers of sandstone in the mode represented by fig. 21, where the 
trap is in wedge-form masses, and which penetrated the rock 
subsequent to its consolidation. In the lake Superior district, 
the trap overflowed the sandstone in sheets, which subsequently 
were covered with another bed of sandstone, and then another 
eruption covered the preceding with another sheet of molten 
trap. There were alternations therefore of melted rock and 
sediments; the two processes were going on at intervals,but dur¬ 
ing the same epoch. 
