and their Contact Zones. 29 



the predominant felspar belonged to this class, and of 

 these seven pointed to anorthite. The associated felspars 

 with anorthite were labradorite, oligoclase, or even rarely 

 albite ; there was usually hornblende, and less frequently 

 mica. Very rarely a colourless pyroxene (diopside) occurred. 

 In only two cases I found quartz. In the five slices in which 

 labradorite probably predominated, the associated felspars 

 were oligoclase and albite, together with hornblende, mica, 

 and quartz. 



3. Microcline. 



Felspars which I have classed thus are found in rocks at 

 the lower part of Riley's Creek, at the Long Gully Gap, at 

 the extreme source of the Sheep Station Creek, and at 

 Eureka — that is, in the quartzose diorites and the allied 

 amphibole granites adjoining the contacts. 



The structure of these felspars is usually most irregular, 

 but of an irregularity differing completely from that of 

 group 2. It is an irregularity of structure, both physical 

 and optical. I find these felspars most frequently as 

 crystalline masses of irregular outline, and of larger extent 

 than any of the individuals of the two other groups. It is 

 common that individuals are built up of various incon- 

 gruous portions, and yet form a whole. Thus, in an 

 example which I give from Riley's Creek (Fig. 11) I found 

 that the imperfectly shaped crystal was composed of three 

 unconformable portions, of which one part was not striated, 

 but had only lines of zonal growth. 



In some cases the crystal is composed of two halves 

 (Carlsbad law), one of which may be plain, while the other 

 is striated. 



In some cases the crystal is a single individual, but then 

 almost always showing lines of growth. 



In other cases, as in a slice taken from a rock at Riley's 

 Creek, I observed, besides these instances which I have noted, 

 where the twin (Carlsbad law) is plain as to one-half and 

 striated as to the other, instances where in a simply com- 

 pounded individual (albite law) the striations only extended 

 partly down the crystal, and then passed almost insen- 

 sibly by a curiously-mottled band into the remainder, which 

 was unstriated.* In a somewhat similar example I have 



* It must be fully understood that when I speak of the striation or of the 

 optical character of the felspars, polarised light is implied. 



