OF NORTH AMERICA. 
73 
directions of subsequent dislocations, which, added to the great difficulty of exploring the country where 
they are found, renders the study of them not easy. The localities whore this older system of dislo- 
cation of the LawT-entines may be best observed are: the northern side of Lake Superior, betw'een the 
factories of Michipicoten and the Pic; the northern coast of Lake Huron, between French river. Lake 
Nipissing, and Fort la Cloche; and the line which goes from Lake Simcoe a little to the north of King- 
ston. I consider as of the same system of dislocation, the centre of the group of mountains , of erup- 
tive and metamorphic origin, that forms the highland of Wisconsin and of Michigan, between the Upper 
Mississippi , Lake Michigan , and Lake Superior. I say only the centre of this group ; for I except the 
copper trap of the south side of Lake Superior, the age of which is much more recent, as well as the 
greater part of the dislocations which extend from Wisconsin river to Menomonee river. Most of the 
mountains in the northern part of New, York State, between the Thousand Islands and Lake George, 
are also of the same epoch, as well as two little granitic groups in- the State of Maine, betw-een Moose- 
head Lake and S* John’s river; also the mountains at the north of New' Brunsw'ick, near Bathurst. 
West of the Mississippi , between the Missouri, the Llano Estacado , the Bio Grande del Norte, and 
the Gulf of Mexico, we meet five masses of granitic mountains, the direction and age of which appear 
to coincide with the System of the Lawrentine mountains. One of these masses is situated in the southern 
part of the State of Missouri; another at Little-Rock, where it is traversed by the Arkansas river; a 
third, the most inconsiderable of all, is found 25 miles east of Fort Washita; the fourth forms an ex- 
cellent landmark in the centre of the immense Prairies of the country of the Comanche Indians, and 
is known under the name of Witchita mountains; finally, the fifth and last occupies the north of Texas, 
where it is traversed by the Colorado river between the rivers Llano and San Saba. 
2. Two Mountains and Montmorency system. — The dislocations giving rise to this system took place 
at the end of the deposit of the first group of Lower Silurian; that is, after the formation of the Pots- 
dam group. Its direction, according to the few observations that I have been able to make, appears to 
be approximatively E. 40’ N. and W. 40° S. The beds of the Potsdam group are greatly elevated near 
their junction with the metamorphic rocks, and are often metamorphosed themselves, which gives them 
a very hard, quartzite aspect. The localities where this system of dislocation can best be observed 
are: the environs of Quebec, especially between the Montmorency and Indian Lorette Falls; Mount Cal- 
vary, in Two Mountains County, near Montreal; and Little F'alls, in New' York State. I think the dis- 
locations that have taken place on the north-western side of the mountains at the north of the State of 
New' York , and on some points of the lino of junction of the Silurian and the Metamorphic rocks which 
runs from Wisconsin to Menomonee rivers, in the eastern part of the State of Wisconsin, may be re- 
garded as of the same age. The movement that gave rise to these dislocations w'as much less consi- 
derable than when the Lawrentine system appeared, and was not felt at distant localities; it has only 
modified some parts of the preceding upheaval , by crossing and penetrating it to form small chains ad- 
jacent to this older range of mountains. 
3. Montreal system. — In many localities, and especially at Montmorency aud at Little Falls, the 
beds of the lower part of the Middle Silurian, or Trenton group, are found deposited hoi’izontally on 
very much inclined strata of the Potsdam group, presenting, consequently, very discordant stratification. 
These beds, forming the Trenton group, have also been subjected to dislocations soon after they were 
deposited. Without presenting any great perturbations, or marks of much disturbance and upheaval, 
these dislocations, which took place after the deposit of the lower part of the Middle Silurian , are, never- 
theless, very clearly marked, and have left very remarkable traces, especially in Lower Canada. The 
summit of the mountain that overlooks Montreal is formed of dykes of greenstone or trap , which have 
entirely crossed the beds of the Trenton group, and are even spread over them. Several other dykes 
of trap which are found in the same position on different points of the borders of the Ottawa river, 
as well as the mountains of Beloeil, Rougemont, Montanville, and Johnson, near the rivers Richelieu, 
Huron, and Yamaska, appear to me to belong to the same system of dislocation, whose general direc- 
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