74 FEANK D. ADAMS ON THE 



Origin of the Drift penetrated by the Tunnel. 



It is evident from the character of the drift as above described, that with the excep- 

 tion of the comparatively few boulders of Laureutiau gneiss embedded in it. it has not 

 been brought from the far north, but has been derived very largely, if not exclusively, 

 from the wear and tear of the Huron shales and rocks of the " Waverly Group " which 

 occur in the immediate vicinity. This is indicated by the character of the gravel which 

 is present in almost every sample of the clay, sometimes constituting a large proportion of 

 the whole. 



Although the peculiar character of the sand shows that it has been derived origin- 

 ally from the gneisses of the Laureutiau, it seems pretty certain that its proximate origin 

 is to be found in the " Waverly " sandstones, since not only are fragments of sandstone, 

 which, as shown above, there is every reason to believe belong to this group, scattered 

 through the drift and occur even in these very beds of sand, but the sandstone itself con- 

 tains nearly all the minerals found in the sand, and among them some of the rarer and 

 more characteristic species. The character of the clays also points to a similar origin. 

 Since the general movement of the drilt in this district was from north to south, the 

 material constituting the drift penetrated by the tunnel must have come from some por- 

 tion of the area now occupied by Lake Huron. 



This conclusion, as will be seen by consulting the accompanying geological map, is 

 in perfect accord with that indicated by the composition of the drift itself, namely, that 

 it is derived from the wear and tear of the Huron shales and beds of the " Waverly 

 Group," seeing that a considerable portion of the southern half of Lake Huron lies in a 

 depression scooped out of these formations. A great part of Lake Michigan also must be 

 underlain by rocks of this age. 



The Leda Clay. 



As above mentioned, the Erie and Saugeen clays do not extend down the valley of 

 the St. Lawrence below Brockville. East of this point their place is taken by a deposit of 

 clay quite different in character and known as the Leda clay. This clay is in places 

 highly fossiliferous, and, as indicated by its fossils, is of truly marine origin. 



For comparison with the clays above described a typical specimen of this Leda clay 

 was examined microscopically. The sample, which was given to me by Sir William 

 Dawson, was taken from an excavation on Sherbrooke street, in the city of Montreal. It 

 is an impalpably fine clay with no admixture of sand and gravel. When powdered and 

 carefully washed a miniite residue remains, which, when examined under the microscope, 

 is seen to consist of little angular fragments quite clear and fresh of the following min- 

 erals: — quartz, microcline, orthoclase, plagioclase, hornblende, garnet, calcite, apatite (?), 

 tourmaline (?). The quartz sometimes shows the uneven extinction so often seen in gneiss. 

 Under the microscope the rest of the clay is seen to consist of exceedingly finely divided 

 kaolin, with here and there a few minute brilliantly polarizing fragments of one or other 

 of the above mentioned mineral species. 



This difference in the character of the Leda clay and the clays from the St. Clair 



