438 EECENT WOEK OP THE GEOLOGICAL SIJEVEY 



highly probable that the deformation of this Archaean mass was- 

 mainly effected by the Post-Lower-Silurian movements. 



Prom Bettyhill to Kirktomy there stretches a belt of flaggy 

 micaceous gneiss, resembling part of the Moine gneiss ; but to the 

 east of the latter locality lies an area of undoubtedly Archaean rocks, 

 several miles broad, which have only been slightly affected by the 

 Post-Lower-Silurian movements. They consist of coarse hornblendic 

 and micaceous gneiss with bands of diorite and gabbro, similar to 

 the Pre- Cambrian crystalline rocks at Cape Wrath. Though the 

 area to the east of Strathnaver has not been mapped in detail, we 

 are at present inclined to believe that this broad belt of Archaean 

 rocks resembles the mass of Pre-Cambrian strata at Strathan 

 Skerray in its mode of occurrence. The latter, as we have shown, 

 is intercalated in micaceous lloine schist or flaggy gneiss. 



The Archaean rocks east of the Naver are traversed by dykes of 

 pink and grey granite, which have been converted into granitoid 

 gneiss by mechanical movements. 



After the Moine schists and gneiss to the south of Tongue had 

 acquired their present strike, dip, and lithological characters, in con- 

 sequence of the terrestrial movements after the Lower Silurian, 

 period, the great sheet of syenite now constituting Ben Loyal was 

 erupted, mainly along the foliation-planes. On the western slope 

 of the mountain the schists and gneiss plunge underneath the 

 intrusive mass with an E.S.E. dip, while along the north and 

 south margins of the area they also pass underneath it. To the 

 east of Ben Loyal the main body of syenite divides into several 

 branching sheets, which are likewise intruded more or less along the 

 foliation-planes. The boundary line traverses the western slope of 

 Ben Loyal at a height of about 1000 feet, while the peak rises to a 

 height of 2501 feet, so that this great intrusive mass is upwards of 

 1500 feet thick. 



Other intrusive igneous rocks pierce the micaceous flagstones of 

 the Moine series, consisting of dykes of diabase and mica-trap. The 

 latter have been followed for miles through the Cromalt Hills, and 

 in one case a dyke traverses both the Moine schists and the under- 

 lying Silurian strata of the Knockan cliff. 



It is obvious that the facts now brought forward furnish a large 

 amount of evidence in support of the theory that regional metamor- 

 jphism is due to the dynamical and chemical effects of mechanical 

 movement acting alike on crystalline and clastic i^ocTcs. It is further 

 obvious that regional metamorphism need not be confined to any 

 particular geological period, because in the J^orth-west Highlands 

 it occurred on a ^ast scale both in Pre-Cambrian time and at 

 some period subsequent to that in which the Durness limestones 

 were deposited.. 



6. Denudation of the Land-surface hefore the time of the 

 Old Red Sandstone, 



In the neighbourhood of Tongue remarkable evidence is ob- 

 tained regarding the denudation of the old land-surface before 



