﻿GEOLOGY OF THOUSAND ISLANDS REGION 57 



space intervenes. This is due in part to the many miles of Postdam 

 boundary in the region and in part to the scanty glacial deposit and 

 hence abundant rock exposures. Many of the exposed contacts 

 are on slopes, and on limestone, and it is these that are most unusual 

 and interesting. 



Plate 8 shows Mr Eaton's photographs of a contact on quartz 

 schists, i mile southeast of Redwood on the Rossie road, a contact 

 already described and figured by Smyth. The contact here is on the 

 summit of a ridge of quartzite, hence is fairly horizontal where 

 photographed, though the level drops away on each side at no great 

 distance. 



Two fine examples of contacts on slopes occur within the limits 

 of the village of Theresa, along with others almost as good. One of 

 these is by the roadside a short distance west of the upper bridge. 

 A high Potsdam cliff borders the roadway for a few rods, with the 

 base of the formation well below the road level. At the west end 

 the base comes up to the road level, the cliff sets back some 20 feet, 

 and the base rises sharply to from 12 to 15 feet above the roadway, 

 exposing impure Grenville limestone underneath. The recess faces 

 north, and is beset with shade of trees so that satisfactory pho- 

 tography is difficult, the view shown in plates 9, 10 being unsatisfac- 

 tory. The surface of deposit has an angle of slope of 45 ° or more, 

 and the soluble limestone has been somewhat eaten away from 

 beneath the sandstone, so that several square yards of the actual 

 basal surface are exposed. This is set with occasional quartz peb- 

 bles, but these are sparse, and except for them the rock is quite like 

 that above. The sandstone is very massive and irregularly bedded, 

 with a semblance of parallelism to the floor of deposit as is usual 

 with the basal Pbtsdam hereabout. 



The other contact mentioned is exposed on the north side of the 

 river just above the lower bridge. The map shows a small Potsdam 

 outlier there, whose narrow, southwest edge appears as a low cliff by 

 the roadside [pi. 11]. The ground level falls toward the river and 

 at the south end of the cliff the base of the sandstone is exposed, 

 resting on Grenville limestone underneath. Plate 10 is a photo- 

 graph of this contact. At the south the cliff bears sharply away 

 from the road and by turning into the yard of the first house to the 

 south a fine exposure of the south margin of the outlier is obtained, 

 showing the Grenville limestone rapidly rising in altitude and car- 

 rying up the Potsdam base with it. The limestone surface falls not 

 only to the west but also to the north. As in the previous case a part 



