88 Analysis of Labradorite Felspar 



at Sky frequently resembled labradorite. I have seen many 

 specimens from that island to which the latter name must 

 be applied. The mineral, which gives a porphyritic aspect to 

 some of the pitchstone of Arran, appears to be glassy felspar ; 

 as far, at least, as I can judge, from having examined a very 

 small fragment of it. The porphyroidal traps, on the western 

 boundary of the Scotch coal district, more frequently owe their 

 structure to the presence of labradorite than to that of felspar. 

 This last mineral, with its usual characters, very rarely occurs 

 in them. We frequently indeed find a white, opaque, foliated 

 mineral, destitute of lustre, which has generally been called 

 felspar; but I have procured both lime and soda from this 

 substance ; therefore it has, in some cases at least, been impro- 

 perly named. The trap-rocks appear to contain other minerals 

 which resemble felspar in many of their characters, but differ from 

 it in composition. One of these, which I found in the neighbour- 

 hood of Stirling, is of a red bright colour, opaque, foliated, and has 

 a silky lustre ; from the small quantity of it which I could procure 

 I could only determine, that it contained a good deal of perox- 

 ide of iron, and a little lime. The analysis of another was pub- 

 lished some time ago by Dr Thomson, who called it Mornite. 

 It is very remarkable that the composition of mornite is precise- 

 ly the same as that of the first specimen of labradorite that I 

 examined, excepting that it does not contain an alkali, but it 

 contains a quantity of protoxide of iron, exactly equivalent to 

 the soda in labradorite. The external characters of mornite and 

 labradorite must also be very similar, for the mineral dealer from 

 whom Dr'Thomson procured the former, when I showed him my 

 specimens of the latter, immediately called them mornite. I have 

 not seen this mineral, which was found in a trap-rock in the north 

 of Ireland. The granitoidal traps of the Scotch coal district, 

 on the contrary, very frequently present well characterised crys- 

 tals of felspar, and rarely labradorite. They often, however, 

 contain a white mineral, which, through a great extent of rock, 

 does not present any of the characters of felspar, but rather re- 

 sembles a zeolite ; and these differ very much, in all their cha- 

 racters, from those which contain felspar. It would be easy to 

 arrange the granitoids, from their external characters, into three 



