36 Diorites and Granites of Swifts Creek, 



more basic rocks, of which it forms a large ingredient and 

 wherein quartz is absent, it occurs either in ill-defined prisms 

 or in large crystalline masses which often have a predomi- 

 nant cleavage apparently in the direction ii. The colours and 

 structure of these two varieties also differ. The former are 

 often glassy in appearance, and yellowish or dark-green in 

 colour ; the latter are in shades of green, often very pale, 

 and in some cases redish-brown. In the latter it is almost 

 invariable that they include exceedingly numerous, imper- 

 fectly-formed crystals of plagioclase, whose outlines are 

 usually rounded as if from incipient re-solution. There are 

 also numerous spheroidal masses of what seem to have 

 been " ground mass," 



It is almost always possible to trace prismatic cleavage 

 in some of the sections across the prism, excepting in those 

 crystalline massive varieties which form almost the whole 

 rock mass, and in . these it is often very obscure, or even 

 absent. I shall refer to this again in describing the extreme 

 form of this rock. In all cases which I have observed, even 

 in those which depart most from the usual appearance of 

 hornblende, the slices are dichroic, and this character 

 decreases with the diminution of the depth of colour and 

 with the increase of the angle of obscuration of the slice. 

 In those varieties which are crystallised in more or less well- 

 marked prisms, and which are glassy in appearance and 

 somewhat dark in colour, the angle of obscuration, accord- 

 ing to a number of measurements in slices approximately 



parallel to ii, were from 11° to 16°; while in the second class 

 of amphibole, which was usually in plates or in crystalline, 

 cleavable masses, having lighter tints of colour, the angles 

 measured were very high, being from 17° to 30°. These 

 angles, the faint dichroism, and the marked cleavage in the 

 ortho-diagonal direction, would lead to a suspicion of diallage, 

 but in many cases I was able to observe the prismatic 

 cleavage of hornblende. In the former class twinning, accord- 

 ing to the usual hornblende law, was not very frequent; 

 and I found one instance where there was no trace of 

 the prismatic cleavage, while yet the prism itself had the 

 angles of hornblende, and not of augite. In the second 

 class of amphibole, twinning is still rarer. Among the few 

 peculiarities of structure to be noted is one which I have 

 frequently observed in the hornblende of other rocks — as, 

 for instance, of the hornblende porphyrites of Bulgurback. 



