TETROGRAPHY OF ONTARIO S5 



margin, but in the centre is so crowded with brownish or greenish particles arranged 

 in rows parallel to the longest axis of the crystals as to be almost opaque even in the 

 thinnest sections. 



The augite is surrounded by a double zone of decomposition products, the outer one 

 dull grey, the inner one transparent and made up largely of fibres (perhaps of quartz and 

 hornblende) arranged radially around the kernel of sound augite (Plate I, fig. 2). Similar 

 zones surround almost all the other minerals, such as biotite, apatite, and even pla- 

 gioclase. Some crystals which appear to be augite are crossed by irregular bauds of 

 magnetite in a way suggestive of olivine, and even the optical characteristics sometimes 

 resemble those of a rhombic mineral. Unless these are really olivine, that mineral seems 

 to be wanting in our diabases. 



B. 



SCHISTOSE. 



Turning now to the schistose basic rocks, we find very few specimens belonging to 

 the augitic group. 



Diabase Scliist. 



But one example was studied under the microscope. This is very fine grained, light 

 green-grey, and composed of minute grains of c^uartz, probably some felspar, a little green 

 hornblende, and much badly weathered augite in dirty grey fragments or prisms. Its 

 cleavage and optical characters make its nature certain. A little titanite is found as an 

 accessory. 



Scapolite-DiorUe-Schist. 



Before considering the hornbleudic schists proper, an interesting group of rather 

 doubtful position may be described. As they present an unusual combination of minerals, 

 they will be treated somewhat in detail. 



Several handsome coarse-grained rocks of green-black hor]iblende, intermingled with 

 violet-grey portions, shewing here and there the striations of plagioclase, were collected 

 as diorites. They are massive in appearance, but often shew traces of a roi;gh parallelism 

 of their constituents, so that they should probably be referred to the schists. 



Microscopic. — Under the microscope, the hornblende proves to be of the usual dark 

 green, non-fibrous kind, and needs no particular mention. 



Plagioclase. — The lighter portions are found to consist i^artly of iilagioclase of a type 

 resembling that of the diabases. It is slightly dusty in appearance, and of varying com- 

 position, as suggested by the angle of extinction from the twin planes. A variety ranging 

 near 4|° is not very plentiful ; another with about 21° is the most frequent ; though a 

 third of 31", perhaps from twin planes according to another law, is not unusual. This 

 indicates albite or andesine and labradorite. The plane of the twin striations, as in the 

 diabases, is often bent, and there is a tendency for a crack to appear at the ajtex of the 

 bend, possibly evidence of some strain or movement in the rock since its formation. 



