and their Contact Zones. 51 



in spaces between the other minerals. The sample taken 

 from the most altered part of the dyke had a very hard, 

 compact texture, and a reddish colour. I found it extremely 

 difficult to prepare, in consequence of its great hardness and 

 toughness. The slice showed me that it is a confused mass 

 of flaky and fibrous aggregates. Some are radiating, some 

 only divergent. Ores of iron have been extensively de- 

 posited between the flakes and fibres, most frequently amor- 

 phous, but also in octahedral crystals, and here and there 

 in the mass are quartz grains. When it is possible to examine 

 the fibres optically I found them to become obscured where 

 the longer diameters are parallel with the plane of polarisa- 

 tion of one of the crossed nicols. The appearances seen in 

 this rock resemble serpentinous alterations ; but they are not 

 pseudo-morphous. It appears to be a case of complete 

 molecular re-arrangement of the rock with, perhaps, abstrac- 

 tion of some of the constituents. This dyke, in its original 

 state, was evidently a crystalline granular hornblende and 

 mica diorite. 



Porphyritic diorite only occurs in one place, as I have before 

 stated. A slice of the dyke showed me a crystalline granular 

 ground mass, in which are many small, dark-coloured, not very 

 dichroic flakes of hornblende. There are also minute ao-ore- 

 gates, which I suspect to be felspar granules. Yiridite is 

 either disseminated, or occurs in patches, having a somewhat 

 radial structure. In this ground mass are laro-e dark-brown 

 glassy porphyritic crystals of hornblende. 



The slices of this hornblende are not very dichroic, are 

 much fractured, and occasionally twinned. There is but 

 little iron ore. The hornblende is N very fresh and un- 

 altered. It occurs in the form of I, ii, o. Besides these 

 porphyritic hornblendes there are also porphyritic patches 

 of alteration products, perhaps representing felspars, which 

 are otherwise not present. 



Amphibolites. — Dykes of this class occur principally at 

 Riley's Creek, and bear much resemblance to the crystalline 

 granular diorites, in which the amphibole preponderates to 

 the almost exclusion of the triclinic felspars. A sample from 

 such a dyke at the lower part of Riley's Creek, near the con- 

 tact, is coarsely crystalline, and inclining to olive-green in 

 colour. A thin slice showed me that the principal con- 

 stituent is amphibole in not very perfect prisms, cross sections 

 of which show the characteristic cleavage. It becomes 

 translucent in shades of green or yellowish-green, and the 



