in Sutherland, Ross and the Hebrides. 355 



but also that the amount of disturbance in the latter is proportioned to the 

 degree of proximity of the former. Hence we might expect, that where the 

 granite disappears, a more full development of the secondary strata would 

 take place, — and such is the case in this district ; for with the recession of the 

 granitic ridge, the regular formations of the oolitic series, from the sandstone 

 of the calcareous grit down to the base of the inferior oolite, are laid open, 

 including the coal-field of Brora*. 



Thin bands of primary slaty rocks are occasionally interposed between the 

 granite and the secondary deposits along the inland boundary of the vale of 

 Brora. These consist chiefly of a quartzose gneiss, and their occurrence in 

 the chasm above Clyne Kirk has already been noticed f: they have since 

 been observed in the bed of a small rivulet which falls into the river Brora 

 near the loch of that name, where they dip E.N.E. 55°, are traversed by 

 numerous veins of granite, and, as above Clyne Kirk, are here also sur- 

 mounted by an unconformable mountain of red conglomerate. 



Granite of the Sutors of Cromarty. 



In my first visit to these remarkable elevated ridges I was unable to examine 

 their structure in detail, being prevented by stormy weather from disembark- 

 ing at various interesting points under their precipitous and rugged cliffs, I 

 was, therefore, induced to employ nearly all my time in collecting organic 

 remains from those detached beds upon the shore, which have, I hope, been 

 identified with the lias of English geologists. 



I have since ascertained that the portion of the Sutors which consists of 

 primary rock, is chiefly a feldspathose and quartzose gneiss, much foliated, 

 and generally nearly vertical ; but in many situations so decomposed as not to 

 be distinguishable from some varieties of the granite of the Ord of Caithness. 

 This gneiss is associated with subordinate slaty rocks, hornblendic and talcose, 

 and is repeatedly traversed by large and small veins of true granite. 



Many writers have contended (and it seems now to be pretty generally ad- 

 mitted), that the granite must have been in a fluid state at the period when 

 these veins issued from its mass ; and others have further presumed that the 



* When the granite recedes from the coast, the beds assume their natural and low degree of 

 inclination, varying from horizontality to dips never exceeding 20°. In the previous memoir on 

 this district and in the accompanying map, the angle of dip at Dunrobin, Strathsteven, and other 

 places in the immediate neighbourhood of the coal-field, has been rated too high ; whilst, on the 

 other hand, where the granite approaches the coast at Portgower, some of the beds which form 

 reefs on that shore are inclined at the high angle of 80° instead of 70°, as marked in the map. 

 See Pi. XXXI. t Ante, page 295. PI. XXXI. fig. 1,2. 



3 A 2 



