Hobbs—The Correlation of Fracture Systems. 19 
either set are parallel to each other. Usually one set is more 
strongly marked than the other and exercises an important influ¬ 
ence in the decay and disintegration of the rocks, and this in its 
turn affects the contours of hill and valley and determines the 
positions of streams, inland lakes and of the inlets, etc., of Geor¬ 
gian Bay. 
‘ ‘ The dikes which traverse both the Laurentian and Huronian 
rocks of the district and the fissures and lines of crushing wTiich 
occur more particularly in the former, have given birth to some 
of the more striking features of the map. . .” 
In an earlier paper Bell 1 had already shown that many of the 
long and straight valleys within the Archean area of Canada 
now occupied by rivers, lakes, or by inlets of the larger lakes, 
have been formed as a result either of the relatively rapid de¬ 
cay on dikes or on lines of close joints. The valleys now occu¬ 
pied by water are further extended by valleys filled in with 
drift. Examples are Onaping Lake, 30 miles long; Long Lake, 
52 miles in length; and Sepiwesk Lake, which with Nelson River 
forms a trench 96 miles in length. The Mattagami River is 
thus guided for 160 miles, and Lake Temiscaming, 35 miles in 
length, with Deep River, forms a rectilinear trench in places 
more than 2000 feet in depth. Soundings show that the rec¬ 
tilinear inlets of Georgian Bay are extended lakeward by 
straight channels at their fronts. The prevailing joints of the 
region correspond perfectly in direction with these trenches. 2 
The map just published 3 by the Department of Crown Lands 
and covering the area about Lake Temiscaming reveals this 
orientation in great perfection (see Plate III). Dr. C. K. Leith, 
who has done much recent geological work in the district con¬ 
firms the general correctness of this map and the correspondence 
of drainage lines with the direction of the prevailing joints. 
As the map indicates, the prevailing joint direction is about N. 
40° W., with N. 50° E., and N-S the directions next in import¬ 
ance. 
iBell, Robert. Pre-Paleozoic decay of crystalline rocks north of Lake 
Huron. Bull. Geol. Soc. Am., Vol. 5, 1894, pp. 357-366, Pis, 15, 16. 
2 Personal communication from Dr. Bell. 
s Map of part of the District of Nipissing, showing agricultural lands 
surveyed on Lake Temiscaming, Ontario, 1905. 
