106 The Sedimentary, Metamorphic, 



sedimentary rocks. On the eastern side of this small stream 

 there are some very interesting rocks of the Diabase group, 

 which are evidently intrusive into the crystalline-granular 

 acid rocks, and which extend over a large tract, probably 

 not less than a square mile in area. There are several 

 varieties of these rocks, of which I collected samples, the 

 examination of which gave the following results : — 



38. — This rock has a black colour and micro-porphyritic 

 structure, as seen in a hand sample. Under the microscope 

 the ground-mass is found to be composed of — (a) traces of 

 micro-felsitic basis ; (6) innumerable colourless acicular 

 crystals lying at all angles ; (c) very numerous rounded grains 

 of devitrifled magma ; (d) very numerous crystals and grains 

 of iron ore (magnetite or ilmenite), in clusters or groups ; 

 (e) many brown-coloured bladed crystals, which are sensibly 

 pleochroic in shades of brown to colourless. 



The obscuration in some is straight, and in others inclined. 

 The mineral is therefore monoclinic ; and when the slice is 

 examined by a higher objective, it is seen that cross sections 

 have the prismatic angles of amphibol, some with the planes 



oo P only, others with those of oo P and go P go combined. 

 The same examination shows that many of the crystals are 

 spindle-shaped, or perhaps with very steep pyramidal planes, 

 the ends of most being ragged. This mineral is clearly an 

 amphibol. It is the largest of the constituents of the 

 ground-mass, and one of the most numerous. There are 

 finally (/) minute grains of yellowish augite. 



In this ground-mass are porphyritically — (g) a few com- 

 pletely serpentinised olivine crystals ; (h) prismatic crystals 

 of augite, which are almost colourless and non-pleochroic. 

 The extinction angle is as high as 40°. Groups of 

 crystalline grains of the same augite also occur. Both the 

 olivine and the augite are quite free from inclusions, (i) Very 

 irregularly-formed felspars, as to which all that can be said is 

 that they are triclinia This rock is an "Olivine diabase 

 porphyrite, with accessory amphibol." 



A second sample I found to have the following com- 

 position. The ground-mass is much altered, but it can 

 be seen to be made up in great measure of minute prisms 

 and fragments of felspar. They are extended in the direc- 

 tion ofthe edge ocP oc (100)- ocP go (010). Intheleast altered 

 individuals I observed that the obscuration angle is high, 

 indicating, probably, a Labradorite felspar. 



