74 



sizes of the porphyry dike. These fragments range in form 

 from angular to sub-angular and rounded. Both the green- 

 stone and the porphyry, but more especially the latter, show 

 characteristic torsion cracks. 



This contact and others in the district, between the 

 Cobalt series and the older rocks, have a striking resem- 

 blance to those which have been described as existing 

 between members of the pre-Cambrian, Torridonian and the 

 older Lewisian, of the Northwest Highlands of Scotland. 

 "The observer may climb one of these Archaean hills, follow- 

 ing the boundary line between the Lewisian rocks and the 

 younger formation, and note, step by step, how the sub- 

 angular fragments of hornblende-schist that fell from the 

 pre-Torridonian crags are intercalated in the grits and sand- 

 stones, thus indicating the slow submergence of the old land- 

 surface beneath the waters of Torridonian time."* 



" The basal breccias which often flank the buried mount- 

 ains are, as already explained, of the nature of scree 

 material. They consist of fragments of the local rocks 

 embedded in a sandstone matrix. The conglomerates, on 

 the other hand, are probably torrential deposits brought 

 down from a district very different in geological structure 

 from that of the area in which the Lewisian gneiss occurs."f 



Slate-uke Greywacke. 



Normally, the basal conglomerate and breccia pass grad- 

 ually upward into fine-grained, delicately banded, slate-like 

 greywacke. The components of the graywacke are so fine 

 in grain that they cannot be distinguished except by exam- 

 ination of thin sections under the microscope. When thus 

 examined, they are found to consist, for the most part, of 

 angular fragments of quartz and feldspar, which is usually 

 quite fresh and undecomposed. The feldspar consists of 

 orthoclase, microcline and the more acidic soda-lime 

 varieties. Grains of glassy volcanic rocks, and of iron ore 

 and other material, have also been observed. Chlorite and 



*The Geological Structure of the North-West Highlands of Scotland, 

 p. 4. Memoir of the Geological Survey of Great Britain, 

 fldem, pp. 286-7. 



