320 



J. W. JTJDD ON THE AXCIEXT VOLCANO OF 



the unaltered to the most highly metamorphosed condition in Scotland 

 and Hungary, and of the light which they throw upon the important 

 question of the origin of foliation. At the present time I propose only 

 to notice the interesting relations of the rock called aplite, which ap- 

 pears to be the final product of metamorphism of some of the Triassic 

 rocks. Upon the old theory that the granitic mass of Hodritsch was of 

 ancient date, and that the Dyas and Trias strata were deposited upon 

 it, the relations of the aplite rock appeared to be altogether inexpli- 

 cable ; while the views of the relations of the igneous, metamorphic, 

 and sedimentary rocks of the Schemnitz area which I have endea- 

 voured to enforce in the present paper suggest, as I hope to show, 

 a remarkably simple explanation of the phenomena, which I think 

 will commend itself to the minds of all geologists. 



The aplite (a variety of granulite) is a crystalline rock composed 

 of orthoclase and quartz, to which hornblende, tourmaline, and other 

 minerals are sometimes added. It always occurs at the junction of 

 the " syenite " and gneiss, and while it sometimes forms masses 

 having a general parallelism with, and graduating insensibly into, 

 the latter, it is not unfrequently found sending off veins into the 

 gneissic rocks, and thus behaves like an intrusive rock, as von Lipoid 

 has well pointed out (see fig. 4). 



Pig. 4. — Aplite forming a vein in Gneiss at Schuttrisberg. 



Aplite. 



>. Gneiss. 



At first sight the occurrence of this granulite, or " semi-granite " 

 (as it is well called), at the junction of the granitic and gneissic 

 masses might be thought to lend some support to the views enter- 

 tained by several geologists of the metamorphic origin of the granitic 

 rocks themselves. But a more careful study of the conditions here 

 presented will be sufficient to convince us that, here at least, any 

 such doctrine is altogether untenable. Microscopic and chemical 

 examination alike demonstrate conclusively that the aplite cannot 

 possibly be regarded as an intermediate stage between the gneiss and 



