GEOLOGY OF TJIK ST. CLAIR TUNNEL. 73 



found to measure "05 x '027 ram. They have a high index of refraction and strong 

 double refraction, with parallel extinction, and show strong pleochroism in tints ranging 

 from light brown, yellowish or greenish brown to deep brown or nearly black. The 

 greatest absorption is in a direction at right angles to the length of the crystal. It is 

 uniaxial and negative and shows no cleavage. These characters leave but little doubt 

 that the mineral is really tourmaline. 



The garnet is pink in color and isotropic with high index of refraction. 



The calcite is present in considerable amount, the sand effervescing readily when 

 treated with dilute hydrochloric acid, and occurs principally as cleavage fragments. It 

 is uniaxial and negative, and between crossed Niçois shows the characteristic white of 

 the higher orders. Although here referred to as calcite, as some of the grains at least are 

 attracted by the electro-magnet showing that they contain some iron, it may really be 

 dolomite in part. 



Pyrite occurs in numerous little yellow grains with crystalline faces. Some of 

 these are very minute but perfect octahedra. In No. IV, in addition to little rounded 

 grains resembling minute concretions, a number of little octahedra were found often 

 in groups. One very perfect little octahedron, showing also planes of the pentagonal 

 dodecahedron, measured '07 mm. in diameter. On account of the good form and sharp 

 angles usually possessed by this pyrite it seeds probable that it has crystallized in 

 the drift. 



The mineral referred to as sphene (?) occurs in small rounded grains with high index 

 of refraction and high double refraction, and is either sphene or zircon, but I was unable 

 to find any grain in which an axial figure could be obtained. 



The fine material deposited from the first washing was found to be made up alto- 

 gether of minute grains of different minerals. In addition to those already mentioned 

 biotite and a colourless mica resembling muscovite, but probably belonging to some 

 hydrated species, were also present. A very small amount of a fine grained aggregate, 

 probably kaolin, was also seen in the slides. 



I'etrographical Character of the Clay. 



A careful microscopical examination of the clay separated by washing from Nos. I. 

 and VI. was also made. These clays were found to be almost identical in character, and 

 may bo taken as representing the clays of the whole deposit. 



The material is exceedingly fine and requires a very high power for its examination. 

 Minute fragments of calcite, orthoclase and little mica shreds can always be recognized, 

 as well as in some cases little grains of plagioclase, hornblende and quartz. Kaolin is 

 also present, and in No. I. occurs in large amount. Mixed with these minerals in No. I. 

 and in No. IV., constituting a very large proportion of the clay, are dark fragments, 

 which are opaque, except on the thinnest edges, where they are seen to be composed of a 

 kaolin-like aggregate holding opaque grains. This is in part at least the Huron shale in 

 a finely comminuted condition, while a portion of it seems to be a decomposition product 

 of orthoclase. 



Sec. IV, 1891. 10. 



