OBSERVATIONS ON THE STRUCTURE OF ROWLEY RAG. 263 
grey veins in such a good state of preservation that by both 
chemical analysis and microscopical examination it was 
possible to determine with facility and comparative certainty 
the relation which the veins bear to the mass of the rock. 
The particular vein which has furnished my specimens was 
about half an inch thick ; it traversed a great detached block of 
stone, so that the extent of it could not be determined. When a 
thin slice is examined microscopically it is seen to consist of 
a network of beautifully clear, apparently quite fresh and 
unaltered crystals of felspar of much larger size than those 
which occur in the part of the rock in the immediate 
neighbourhood of the vein. The angular spaces among 
these are filled up with a clear colourless substance, which in 
many parts swarms with brownish dust (as seen with a low 
power), and is not altogether free from it in any instance. 
A few crystals of green augite, contrasted with the much 
browner augite of the normal rock, will also be observed, and 
a very small quantity of green and brownish fibrous materials, 
evidently the products of decomposition. 
When polarized light is employed the felspar is found to 
be exclusively in either simple or only singly twinned crystals; 
the multiple twinning so characteristic of the triclinic 
felspars is, so far as I have observed, totally wanting. This, 
however, is not a positive proof as to the nature of the 
mineral, but the presence of ortlioclase is proved by the fact 
that many crystals will be found on careful search which 
“ extinguish ” when their length is parallel to the principal 
planes of the Nicol prisms, and many of the twinned sections 
become dark in both halves together when in this position. 
As to sections which do not fulfil either of these conditions, 
they may be ortlioclase or they may be triclinic. The 
chemical analysis of the vein also shows such a percentage 
of potash that from this alone we might have inferred the 
presence of ortlioclase. 
The felspar also occurs in the form of extremely slender 
crystals embedded in the dusty material previously mentioned, 
and these very fine blades are often slightly curved. 
Still using polarized light, we shall find that most of the 
spaces between the felspars remain quite dark between 
crossed prisms in all positions ; that, therefore, we have to do 
with a glassy residuary base. In a few places, however, this 
has a stringy, uneven look, coming near, I presume, to 
Eosenbuscli’s microfelsite, and in some others there is a very 
faint and vague polarization, evidently due in some cases to 
very fine films of felspar, but in others with a quite decided 
cryptocrystalline character. 
