
266 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
considering the well-defined crystals of the white felspar of these veins to be 
as thoroughly good a species as the accompanying flesh-coloured orthoclase. 
A similar association with orthoclase is also to be seen in the veins— 
whether intrusive or exfiltration—which occur in hornblendic gneiss,—as at 
Rispond and Geo-na-Shermaig. 
And I consider that it is even more singularly clear as to the last of the 
felspars—that which is generally held to be the most doubtful. 
Andesine is by many held to be merely an altered oligoclase ; but in Scotland 
oligoclase isa granitic felspar, andesine is a gneissic one ; and it occurs only in, 
or in near proximity to primary limestone. One would have expected that the 
felspar specially pertaining to limestone would be the most calcareous of all; but 
it is not so; with the exception of the limestone locality at Glen Beg, andesine is 
the only felspar I have found in limestone—and it occurs only with limestone. 
In Scotland it cannot, for two reasons, be altered oligoclase :—first, there is 
no oligoclase with it, or near it; and second, it is perfectly fresh, lustrous, 
undecomposed,—and in larger crystals than the oligoclase which occurs else- 
where. 
In Scotland, therefore, the felspars belong so specially each to its own rock, 
that if we can tell the felspar, we may say that we are well on in the determina- 
tion of the rock. 
But can we tell the felspars? That is—can we discriminate between them — 
as they ordinarily occur in rock masses 4 
That they can* be distinguished by the employment of the polariscope, 
according to the method established by DEscLoIsEAux, we know; but the 
bulky polariscope could not be employed in the field, even supposing that the 
obtaining plates sufficiently delicate for that mode of investigation were an 
effort of ordinary skill, which is very far from being the case. 
We have the old physical distinction of strzation,—laid down as infallible in 
separating the ortho from plagio-clastics ; let us see if this be an infallible test, 
or what value is to be assigned to it. 
No twinning of orthoclase has as yet been shown to be so repeated as to 
exhibit the phenomenon. 
Does the absence of striation then entitle us to pronounce the felspar to be 
orthoclase ? 
Reverting to the physical descriptions of the felspars analysed, we find that — 
to the eye, or to the lens there exhibited striation—of four albites, one—rarely ; 
of ten oligoclases, four; of four andesines, one; of ten labradorites, three; of 
two anorthites, one; of one latrobite, none; or, altogether in the thirty-one 
plagioclastic felspars, striation was ten times seen, twenty-one times not seen. 
The oligoclase near Lairg,—the crystals of which are perhaps larger than at 
* Except as regards the discrimination of oligoclase and andesine. 

