102 The Sedimentary, Metamorphic, 



The slices cut perpendicular to the planes OP and 



oo P oo showed, as might be forecast, features in conformity 

 with the structure I have described. The main felspar 

 obscured at angles as referred to the trace of the less perfect 

 cleavage of 6° 30' to 9°, thus agreeing fairly with the other 

 observations, taking into the consideration that the slices 

 were not precisely true to the intended direction. The 

 second felspar showed in these slices in much larger amount 

 than in those of either of the other directions, no doubt, 

 owing to the slight angle which the slice formed with the 

 veinlets. It is sharply twinned- in very numerous lamellae, 

 the obscuration angles of which are from 10° to 14° on either 

 side of the composition face. 



In addition to this form of twinning, according to the 

 Albite law, I also observed a number of twinned crystals, 

 seldom compounded of more than two members, which were 

 interposed according to the Pericline law. These crystals 

 occurred singly or several near to each other, and their 

 terminal planes were sharply marked, while the others were 

 usually irregularly bounded by the walls of the veins 

 themselves. 



The only other inclusions which I observed in this felspar 

 were a few small quartz grains, and several small divergent 

 masses of talc plates. This felspar is entirely in. accordance, 

 in its characters, with the felspars which I have collected 

 and examined from similar veins at Omeo, and in other 

 parts of the district. 



The main felspar does not obscure in the basal section 

 parallel to the edge P.M. It is therefore not monoclinic; and 

 this is also further shown by the inclination which I find 

 the cleavage faces always have to each other. It has not the 

 extinction angle usually given for microcline ; but I have 

 observed that in felspars such as this the angle is not a 

 constant one, and I therefore class with microcline all those 

 potassa felspars which are triclinic in form, although their 

 obscuration angles may, as in this case, be less than 15° 

 30'. The second felspar is evidently albite ; and the very 

 small amount of the third felspar may, with fair prob- 

 ability of correctness, be designated as oligoclase. The 

 felspar, as a whole, is a microcline-perthite. 



The abnormal structure of this felspar, as indicated by the 

 variation in the angles of extinction, shows quite clearly 

 how very disturbed the conditions were under which these 



