and their Contact Zones. 53 



instance in which I could obtain measurements of the angles 

 of obscuration in the felspars I found them to be 5° on each 

 side of the composition face. 



A slice prepared from a third dyke in the same neighbour- 

 hood afforded me similar observations, with this exception,, 

 that in this case the colourless pyroxene amounted to perhaps 

 one-third of the constituent minerals, and also constituted 

 the nucleus of two crystals, thus showing that the simul- 

 taneous occurrence of amphibole and pyroxene is a case of 

 envelopment and not of alteration. 



Diabase. — The only dyke of this class which I have met 

 with is found in the schists at a road cutting at lower 

 Swift's Creek. A slice of this dyke I found to have a light- 

 brown, cloudy, crypto-crystalline ground mass, in which are 

 — (1.) Minute prisms of plagioclase, all of which are more or 

 less converted into micaceous aggregates. (2.) Large porphy- 

 ritic plagioclase prisms, in which the alteration is distinctly 

 micaceous. Yiridite has been deposited in these felspars. 

 The micaceous alterations are mostly central, but also in 

 some cases extend to the exterior. The angle of the 

 principal optical sections in the sections of the crystals are 

 from 15° to about 30°, thus pointing to labradorite. If this 

 is so, the alterations must produce, as a colourless micaceous 

 mineral resembling muscovite in its microscopic optical 

 characters, a mica containing soda or lime. In other words, 

 it suggests paragonite or margarite. Augite exists in 

 yellowish crystals, which have, however, been mostly con- 

 verted into chlorite. (4.) Magnetite, and also amorphous 

 iron ores, probably the result of the alterations in the rock. 

 (5.) Cavities filled by colourless carbonates, edged by delessite. 

 This rock is evidently a porphyritic diabase, or, in accordance 

 with the classification of Professor Bosenbusch, a diabase 

 porphyrite. 



(e.) The Contact Metamorphic Rocks. 

 Their Component Minerals. 



Felspars. — In the contact schists, with the exception of 

 one narrow zone of limited >extent, the felspars are of 

 extreme rarity. The triclinic soda and soda-lime felspars 

 are in the minority, and the lime felspar anorthite is entirely 

 absent. 



I have found the soda and soda-lime felspars also as con- 

 stituents of the aplite beds. These immediately adjoin 



