82 ACCOUNT OF A MINERAL FROM ORKNEY. 



External Characters of the Mineral. 



Its colour in the fresh fracture is greyish- white, passing 

 into pale yellowish- white, where it approaches the surface of 

 the specimen, which is weathered. 



It occurs in masses, of various sizes, dispersed, along with 

 lead-glance, in a species of coarse slate, which may perhaps be 

 considered as shale passing into clay-slate *. This rock ap- 

 pears to rest on mica-slate, which is in connection with gneiss 

 and small-grained granite, rocks which occur in no other part 

 of the Orkney Islands constituting mountain masses. The 

 mineral perhaps may occur crystallised, for it has a distinct 

 crystalline texture ; but the specimens I found were so much 

 acted on by the weather, that no trace of crystalline figure is 

 discernible. 



The lustre is shining and pearly in the principal fracture ; 

 glistening and resinous in the cross-fracture. The weathered 

 surface is earthy and dull. 



The principal fracture is radiated, somewhat diverging ; the 

 cross fracture is uneven, approaching to splintery. The mine- 

 ral is translucent where fresh, but opake externally. 



It is soft ; so as easily to be scratched by a knife ; it scratches 

 calc-spar, but not fluor-spar. 



It is rather brittle, and easily frangible. 



It is heavy. Its specific gravity is 3.7, or more accurately 

 3703, to water as 1000. 



Chemical 



* I here state my own impression at the time when the mineral was found : but 

 Professor Jameson, to whose authority I pay the greatest deference, has obser- 

 ved, in a work long ago printed, that the rocks in that neighbourhood consist of a 

 mineral " intermediate between shistose and indurated clay.''' 1 — Mineralogy of the 

 Scottish Isles, vol. ii. p. 233. 



