GEOLOGY 241 



Descending order: 



feet. 



1. Rusty weathering, dark gray, siliceous rock containing 



ankerite (carbonate of iron and magnesia) and mag- 

 netite 20 to 100 



2. Dark gray siliceous rock, containing magnetite with 



small quantities of ankerite 50 to 250 



3. Red jaspilite rich in hematite ore 10 to 100 



4. Red jaspilite poor in hematite ore 5 to 20 



5. Purple or greenish weathering, dark-green graywacke 



shales 10 to 70 



6. Red jaspilite poor in hematite ore to 5 



7. Light greenish-gray sandstone and shale 10 to 300 



8. Fine grained dolomite to 50 



The iron ores have a greater thickness and are richer on the 

 islands in the middle of the chain than elsewhere. 



The rusty weathering, dark-gray siliceous rocks of division 

 I. are found on all the islands from Flint to McTavish, 

 being wanting only on Cotter island. The typical rock is a 

 dark-gray chert made up of finely divided silica showing 

 under the microscope small grains of quartz filled in by 

 later accessions of that material in a finely divided state. It 

 contains minute crystals of magnetite scattered through the 

 mass, and also patches of crystalline carbonates. At the south- 

 ern end of the chain it is cherty and sometimes light-green in 

 colour. These rocks are usually in thin beds, the parting 

 between the beds filled with brownish ankerite, which also 

 occurs in flat lenticular masses inclosed in the cherts ; many of 

 these masses are several inches in thickness and several square 

 feet in area, so that the rock usually contains from twenty to 

 fifty per cent of ankerite. These ores are too much broken and 

 too intimately mixed with the cherts for profitable mining. The 

 rusty character of the rock is due to surface decomposition of 

 ankerite to limonite. The beds increase in thickness as the 

 islands are followed northward, and reach their maximum 



