646 J. J. H. TEALL ON THE CHEMICAL AND 
were separated from the rest and analyzed with the following 
result :— 
UO), ilove salehe sorte uuiawens (ete oes 61:18 
A Oe aa ha ak ae eae 19°95 
HNC) 89.93 ed eiecer a ia dois 3 BW) 
Gla Ors oa vat eewauet Secu Silage 5:45 
ae SOT REN AAPA oe mR RS € aoe 
Eee aia 
SEO ai Ne aaxeies 5-24, snot far is 
99°36 
Both microscopic and chemical analysis shows that we are not 
dealing with pure felspar substance, but a mixture of quartz, felspar, 
and decomposition-products. 
The molecular ratio of Na(K):Ca, according to the analysis, is 
2°18:1; but we must remember that some lime has been removed 
from the rock. Nevertheless, by introducing the most favourable 
assumption as to the amount of lime which has been removed, we 
cannot make the ratio of Na(K):Ca so low as 1:1, and we seem 
therefore driven to the conclusion that in the original felspar sub- 
stance, whether it consisted of one or more species, this ratio agreed 
approximately with that of andesine. The large amount of K,O 
may imply the existence of an independent potash felspar; but of 
this I am not able to give any definite microscopic evidence, and we 
must remember that some andesines are supposed to contain as much 
as 2 or 3* per cent. of this substance. 
The dominant Pyrowene—This mineral, in the thin sections and 
in a perfectly fresh condition, appears to be of a pale brown colour. 
In the finer-grained varieties it occurs in the form of grains, and in > 
the coarser varieties either as grains or irregular plates. The usual 
prismatic cleavages are common, and twinning may not unfrequently 
be observed. External crystalline boundaries are, as a rule, absent. 
In the large irregular plates a fine striation may frequently be ob- 
served. This, however, is by no means a constant feature. In the 
freshest rock specimens it is absent; in the most altered specimens 
it is universally present where the pyroxenes attain any consider- 
able size. Some sections of pyroxene show it in certain parts and 
not in others. It seems impossible therefore to avoid the conclu- 
sion that this structure is of secondary origin. 
It reminds one at once of the corresponding structure in diallage ; 
and an interesting question arises as to whether we are here dealing 
with true diallage—that is, with a pyroxene having a laminated 
structure parallel to «a Pa—or with some other modification of 
augite. It has already been pointed out that in the coarsest variety 
of whin the pyroxene is developed in the form of elongated prisms, 
with more or less definite crystalline faces. If, then, this variety 
* Rammelsburg, ‘Handbuch der Mineral-Chemie,’ Band ii. pp. 569-570. 
