Dr Hibbcrt on the jyistrihution 
reach the Point of Bragastef, from whence our circuit first com^ 
menced. 
Before quitting the discussion of these rocks, a few words re- 
main to be said with regard to the general rela ions of sandstone 
or granular quartz. I have hitherto considered this rock as 
possessing the character of a nucleus, since we have exhibited 
its apparent influence in determining the course and direction of 
other strata connected with it. But when sandstone or granular 
quartz assumes a conglomerated structure, its character appears 
to undergo some cliange. This change consists in the course and 
extent of its strata being determined by the presence of other 
rocks, such, as in the instance now given, by the clays tone-por- 
phyry of Papa Stour 
THE GIIANITIC DISTRICT OF SHETLAND. 
The granite of this district is first traced from the north of 
the Isle of Vcmentry ; it occupies the greatest portion of the 
Island of Meikle Rheu, and is inferred to be continuous through 
the channel between Meikle Rheu and Northmavine, from 
the circumstance of forming the insulated rocks named the 
Drongs ; and, lastly, it extends from the south coast of North- 
mavine to tlie Island of Wya, situated to the north of the pa- 
rish. The whole extent of the granite, supposing it to be con- 
tinuous from Meikle Rheu, is 24 miles in length, and from to 
5 J miles in breadth ; it rises to a height of 1470 feet, forming 
Ronas Hill. 
To the east of the granite, in junction with it, is a mass of si- 
enitic greenstone, which may be traced from the Island of 
Meikle Rheu to the south of Ronas Voe. It is 12 miles in 
length, and about 3 in breadth. 
These two masses of granite and rienitic greenstone, mutual- 
ly penetrate each other by extensive processes, one of which I 
traced a distance of five miles ; at the same time, insulated masses 
of granite occur in the sienitic greenstone, and vice versa. 
Strata and Rochs connected with the Granite and Sienitic Green^ 
stone of Northmavine. 
The line of junction between the granite and sienitic green- 
stone is not, however, complete. A space, in one instance, is 
* For several interesting details on the rocks of Papa Stour, see Dr Fleming’s 
Account in the first volume of the Wernerian Transactions. 
