28 Diorites and Granites of Swift's Creek, 



are peculiar and interesting. The compound individuals 

 are often seen to be well crystallised ; but also, on the other 

 hand the converse is frequently the case. 



In some slices the larger individuals are all of imperfect 

 outline. But in all cases the structure is either highly com- 

 pound or irregular. 



The compounding takes place almost always according to 

 three laws, and the lamellae are disposed most irregularly. 

 The two halves of the twin are rarely even approximately 

 the same in mode of composition. Instances of such struc- 

 ture are given in Figs. 1, 2, 3, 4. 



The peculiar structure of these felspars, their broad bands 

 of vivid colour by polarised light, and their optical charac- 

 ters, sharply distinguish them from those of the first group, 

 which, as compared to them, is characterised by regularity 

 of structure. 



In this group the smaller individuals are usually most 

 completely formed ; but there are many cases where these 

 felspars are only twinned granules, similar in character to the 

 instances figured Nos. 7, 8, 9, 10. 



As examples of well-formed crystals I give Figs. 5, 6. 

 The latter is a good illustration of the concentric and not 

 always concordant growth of these felspars. 



The rock at Sheep Station Gap, which is mainly composed 

 of these felspars, is almost wholly decomposable in fine 

 powder by cold concentrated hydrochloric acid. In order to 

 apply further tests in the absence of a quantitative analysis 

 I did as follows : — A slice taken from the very f elspathic 

 rocks just mentioned showed me only well-marked felspars 

 of this group having angles of obscuration of from 30° to 45°, 

 and with all the peculiarities of structure which I have 

 noted. 



I placed this slice for eight days in cold concentrated 

 hydrochloric acid. The felspars were attacked, and became 

 entirely opaque and cloudy. No striations were then visible 

 by polarised light ; but there were in two or three cases un- 

 striated or slightly striated margins which had perfectly 

 resisted the action of the acid. There were also a few small 

 felspars in the mass equally unaffected, and in one of which 

 I observed the optical section to be parallel to the longer 

 diameter. These observations point to the felspars being 

 mainly anorthite, with albite or oligoclase, either forming a 

 margin to the former or occurring alone as small crystals. 

 Of the thirty-two selected examples I found twelve in which 



