482 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
Insoluble silica, 3°286 per cent. ; possible impurity, Delessite—the rifts | 
being sometimes lined therewith. : 
I have. also obtained the same substance at the Giant’s Causeway 
in Ireland, both in the basalt and in a bed of red clay; (this is the 
“plynthite” of THomson, which I find to be bole, and possibly was a 
bed of overflowed and calcined mould). More lately I have obtained it 
in the basalt at Kinkell, along with black lustrous hornblende, sanidine, 
and Delessite. 
From Volcanic Rocks in Old Red Sandstone. 
20. This specimen was forwarded to me by Professor GeErxiz. It was 
of a dark bottle green colour, and high vitreous lustre; the specific gravity 
was 3° 36. 
1-3 grammes yielded— 
Silica, . ‘ “591 
From Alumina, . -008 
* 599 = 46-076 
Alumina, . é : 5» pee 
Ferrous Oxide, . : i OD. 
Manganous Oxide, . : *461 
Lime, : : : . LO“ OT 
Magnesia, . : : . 15°653 
Potash, < : : ’ ‘818 
Soda, : , : 058 
Water, . d 5 : *38 
Lost in Bath, +142 per cent. of moisture ; probable impurity, a trace of | 
sanidine, or even a more sodaic felspar. | 
The large quantity of alumina in both of these augitic glasses is remarkable. 
In forwarding this augite, November 2d, 1877, GEIKIE writes :— 
“You should receive to-morrow a parcel containing the pieces of the John | 
o’ Groat’s augite. , 
“T have not time to send you full particulars at present, but will do so im 
time for insertion in your paper. It occurs in rough, rounded, and broken | 
lumps, imbedded in the coarse agglomerate of a volcanic rock, belonging to the 
Upper Old Red Sandstone times. 
“T have had it sliced, and have examined it microscopically. It is not im 
the least dichroic, and has none of the other features of hornblende. I have 

