MALCOLMSON OLD EED SANDSTONE, 349 



a little promontory is fonned by the gneiss rising on each side of a 

 dyke of reddish-grey homogeneous trappean rock, approaching to 

 homstone, which is not acted on by acids, and fuses with great 

 difficulty before the blowpipe. It appears to have flowed over 

 the edges of the upraised gneiss, and the continuation of the 

 dyke passes into the body of the hill. A small chasm is worn into 

 the little cape at the line of junction of the upraised gneiss, at the 

 bottom of which and under the trap the usual granite-veins are seen 

 in the gneiss. I soon after examined the dykes in the lias, described 

 by Ml'. Miller some years before (ia the ' Legends of Cromarty ') 

 as basalt, and since by Mr. H. E. Strickland as sandstone-grit ; and 

 notwithstanding the weight of authority in support of this latter 

 opinion, and the fact that fragments of the smaller dykes effervesce 

 in acids and crumble down into a siliceous sand, I am still of opinion, 

 from the manner of their ramifications and the altered state of the 

 limestones and shales through which they pass, that they form the 

 upper part of dykes that penetrate the lias from the eruptive rock, 

 forming the nucleus of the hill, — and whose existence not far beneath, 

 Mr. Murchison long ago inferred from other appearances presented 

 by these very remarkable hills. " If an adequate cause," he says, 

 " be required to explain the great upheaving of the granite upon 

 this coast, may we not seek for it in some deeply-seated volcanic 

 agency, struggling (in vain) to expand its forces from beneath the 

 vast mass of primary rocks, of which the mountains of the N.E. 

 Highlands are composed ? " * 



It only remains to make one or two observations on the relations 

 of the Gamrie Ichthyolites, the examination of which is rendered 

 comparatively easy by IVIr. Prestwich's very clear description of the 

 district f. But the eye of an observer accustomed to contemplate the 

 vast series of strata included in the Old Red Sandstone system at 

 once perceives that there is no ground, in the general aspect of these 

 rocks, to refer them to another foimation ; and I looked in vain for 

 any instance of unconformity of the dip of the strata associated with 

 the Ichthyolites to that of the inferior beds. Striking instances, 

 indeed, occur of the strata on the opposite sides of the faults de- 

 scribed by Mr. Prestvdch having very different dips ; but this does 

 not imply unconformity of stratification, which was not to have been 

 expected even if the Gamrie Ichthyohtes had been part of the coal- 

 measures. The Ichthyolites are associated with strata having a 

 remarkable resemblance to those of Dipple, the coarse conglomerate 

 above and the shales below (in which I found plants the same as those 

 of Lethen) being almost identical in character ; and I obtained from 

 the thin-bedded red sandstone- conglomerate under this shale, frag- 

 ments of tuberculated scales, such as occur in a similar y^ocTc at Tynat, 

 and which can be referred to the singular fossils so abundant in 

 Nairnshire. The series of sandstones, however, below the fossils 

 attain a much greater thickness than in other places to the south of 

 the Moray Frith, in which respect they resemble those of Cromarty. 



* Geol. Trans. 2nd ser. vol. iii. p. 359. t Ibid. vol. v, p, 139. 



