﻿1861.] 



MTTRCHISON AND GEIKIE HIGHLANDS. 



175 



their true succession, he had, even in his day, so faithfully delineated 

 the essential mineralogical distinctions between the gneiss of the 

 "Western Isles and that of large tracts of the mainland on the east, as 

 to prepare the way for those who, like ourselves, have worked out the 

 proofs of a clear order of superposition. Combining these proofs of 

 succession with the manifest distinction in the lithological structure 

 of the two classes of rock, we are of opinion that no geologist can 

 confound the Laurentian or fundamental gneiss with the so-called 

 gneiss of the superior crystalline schists, which, instead of being a 

 massive hornblendic and granitoid rock like the first-formed, is, on the 

 whole, a flag-like, micaceous, and quartzose deposit of very different 

 characters. 



Although we have not, like Macculloch, traced the persistence of 

 this same Hebridian or Laurentian gneiss through all the isles where 

 it abounds, we have observed it in the Isles of Rona and Raasay. It 

 forms the whole of the former island, and the northern end of the 

 latter at Castle Brochel in Raasay; it is overlain by Cambrian shales 

 and sandstones, which, towards the south end of the island, are re- 

 placed by coarse conglomerates, well seen in the watercourses south 

 of the road from Raasay House* to the eastern shore, as well as 

 along the cliffs of the opposite Island of Scalpaf. 



Cambrian Conglomerate of the Lewis. — With the exception of 

 superficial accumulations of sand, clay, or pebbles, the island of 

 Lewis offers nowhere any deposit overlying the ancient gneiss, except 

 an old conglomerate composed exclusively of fragments of that rock, 

 associated with a few sandy layers. These beds are exhibited at in- 

 tervals on both sides of the Bay of Loch Tua. They are made up of 

 pebbles of the older gneiss, varying from the smallest size to boulders 

 larger than any among the Cambrian rocks of the mainland : their 

 chocolate-red colour, precisely that of the sandstone strata of Suther- 

 land and Ross, is probably due to the extensive decomposition of 

 the iron of the hornblendic masses which prevail in the subjacent 

 gneiss. 



As this conglomerate is unknown in the Lewis, except upon the 

 headlands and bays east of StornowayJ, where it is exactly opposite 

 to the Cambrian conglomerates and sandstones of the mainland of 

 Ross-shire, which also are formed exclusively out of the subjacent 

 bottom-gneiss, and dip away to the west, as if passing to the Lewis, 



* Mr. Geikie has explored and mapped the geology of Raasay, with reference 

 especially to its Oolitic strata, and hopes soon to be able to lay the results before 

 the Society. 



t In a new sketch-map of Scotland, now in course of preparation, we have 

 represented the Isles of Tiree, Coll, and Iona as composed essentially of Lauren- 

 tian gneiss, not only because they are identified with the gneiss of the Long 

 Island by the minute description of Macculloch, but also from the independent 

 testimony of the Duke of Argyll, who lately visited his property in Tiree. 

 Thoroughly well acquainted with the character of the so-called gneiss of Argyll- 

 shire, where granites and intrusive rocks equally abound as iu the Hebrides, his 

 Grace so recognized the striking lithological distinction between the two classes 

 of rock, that he declared to us that no such gneiss as that of Tiree exists on the 

 mainland. 



\ The Lighthouse and part of Stornoway stand on this conglomerate. 



