and Igneous Rocks of Ensay. 101 



obscuration passes like a cloud from one part to the other. 

 The limits of this variation in the obscuration angle I found 

 to be about 2° 30' ; for observations varied in different parts 

 of the slice from 6° to 8° 30' as referred to the trace of 

 the second cleavage. 



Numerous veinlets of a second felspar traverse the main 

 mass approximately at right angles to the above-mentioned 

 cleavage, and thus may be considered to agree in position 

 with the macroaxis. This is further shown by their extreme 

 irregularity in width, being in parts mere threads, and in 

 others bulging out into small masses. Some veins bifurcate 

 and others run out. This irregularity of structure is, I 

 think, connected with the difficult separation of the felspar 



in the direction of the macropinnacoid. This second felspar 



o 

 is sharply twinned parallel to the edge OP — ooPoo, and 

 obscures on either side of that direction at angles, as 

 measured in different parts of different slices from the same 

 sample, of from 30' to 3°. 



I have attempted to show the structure of the felspars as 

 I have now described it in fig. 4, Plate IV. 



In the slices prepared from pieces of the second cleavage 

 I found the same two felspars. The main mass obscured in 

 the same partial manner as in the basal sections at angles 

 which differed by 2°. These observations, as referred to 

 traces of the basal cleavage, were from 6° 30' to 8° 30'. 

 Traversing the main mass there are veinlets of the second 

 felspar, which are here also very irregular, both in width 

 and extent, as will be seen from the sketch given in fig. 5, 

 Plate II. As referred to the traces of the basal cleavage in 

 the slice, the inclination of these veinlets was 63° from the 

 direction of the axis c. These veinlets, therefore, follow 

 the direction of the macropinnacoid, and, as I have said, their 

 extreme irregularity conforms with the obscure cleavage in 

 that direction. This second felspar obscured in different 

 parts of the same slice at from 15° to 18°, as referred to the 

 trace of the basal cleavage. 



In one slice I observed some fine lines of a felspar which 

 obscured at a position of the slice different to that of either 

 of the others. I did not find it practicable to measure its 

 obscuration angle, owing to the extreme thinness of the 

 lamellas. It may be, however, conjectured as being oligoclase, 

 to which also the small percentage of Ca.O. in the analysis 

 points. 



