139 
Geological Formations at Rockland, Co. Russell, Ontario. 
System. 
Formations. 
Thickness in Feet. 
Fossil Remains. 
1. Fluviatile 
2. Leda clay 
3. Glacial clay, etc. . 
Various ..... 
I. Post-Tertiary .... 
Rep. to 25 feet 
None observed. 
Here an Unconformity Occurs. 
II. Ordovician . 
4. Trenton 
5. Black River. 
6. Chazy 
7. Calciferous . 
8. Potsdam 
About 50 ft Abundaat. 
75 ft I Not rare. 
" 60 ft Abundant. 
5 ft [None observed. 
75 ft ;ScolithusCa:iadensis. 
Here the Second Unconformity Occurs. 
III. Archaean. 
9. Laurentian . 
Several thousand ft. 
None observed. 
The Laurentian Outcrop. 
The basal beds of the Potsdam formation are seen to lie uncon- 
formably over the denuded and rounded, or irregular surface of the 
Archcean or Laurentian rocks. These consist of rather coarsely crystal- 
line gneisses and mica schists, along with hornblendic rocks, whose 
petrographical relations and characters deserve special study. They 
furnished the material which helped in the building up of the subsequent 
formations, the quartz of the Potsdam sandstones having been 
derived from the granitoid gneisses of this vicinity. 
The Potsdam. 
The Potsdam formation at Rockland Mills forms a more or less irregu- 
lar zone of heavy bedded, light greyish blue or white colored sandstones, 
which at times become glassy in appearance and give the formation a 
truly quartzite character. Nevertheless, the grains of quartz may be 
easily detected, and are cemented together for the most part by silica. 
