PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 269 
Combining Anorthite and Latrobite, we have— 
Latrobite, Glen Gairn, . 7 3 : ' ; : So aS 
Lendalfoot, Ayrshire, . : : F ; : ‘ ae oe On 
Fetlar, Shetland, . : : ; : , 3 : . 38°*099 
Average, 2°87; range, ‘35. 
2°5 2°6 Pail | DR 2°9 35 
505 2°58 
ee 
Orthoclase 
2607 2627 
os aw, 
Albite 
2° 618 Oligoclase 2° 834 
26725 2 37105 
Ne 
Andesine 
2:°672 Labradorite 2°954 
A consideration of the above diagram shows that, as there exists a well- 
defined gap between the specific gravities of orthoclase and the other felspars, 
the former might, failing other distinctions, be recognised by its low gravity : 
secondly, that as the gravities of each of the plagioclastic felspars overlaps from 
some localities those of the others, no one of these species can be individualised 
by the determination of its gravity. 
‘It is interesting, however, to observe the gradual increase of gravity, as the 
_ percentage of silica is diminished, and that of alumina increased, in the different 
| members of this family. 
2 (ok Anorthite 3° 09% 
| 
Gravity having thus failed to lend much aid, we turn to the cleavage angle. 
The use of the reflecting goniometer, in far shorter time than any other process 
of determination, and with unerring certainty, enables us to determine the 
orthoclases. But when we measure such rough cleavages as are to be obtained 
by the splitting up of imbedded crystals of the other felspars, we do not 
| obtain all the aid that we might hope for. 
| Of Albites,—true angle 86° 24’,—there were got— 

Stromay, : ; 86° 21’ 
Nudista, curved, : 86 32 
Colafirth, ‘ ; 86 45 
VOL. XXVIII. PART I. 4A 
