22 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 21, 



preserved their prevalent slight inclination to the N.E. and seemed 

 to be but slightly altered. The prisms of the greenstone being as 

 usual at right angles to the cooling masses on either side, really resem- 

 bled beds (slightly disturbed only) in the general mass of the schist 

 or shale — so much do the two classes of dark rock resemble each 

 other, until the hammer is applied to them — and even then the trap- 

 rock is seen to have assumed to a considerable extent the scaly or 

 fissile character of the Silurian deposit which it bisects. 



On following this dyke from the Nethan to the very summit of 

 Nutberry Hill which it occupies, we observed that its vsddth (varying 

 somewhat in different spots) was marked by the grassy verdant 

 tint of the vegetation above it, as contrasted with the brown colour 

 of the heath on either side of it upon the schist. 



In descending the Logan Water, and at a short distance from the 

 junction of the Upper Silurian and Old Red, a boss of intrusive por- 

 phyry appears, and again red sandstones follow with another and 

 stronger course of porphyry which has a hornblendic character. A 

 conglomerate then appears, succeeded by other red fissile sandy schists, 

 and next red rocks alternating with light greenish or bluish-grey and 

 slightly micaceous sandstones, which are, in fact, intercalated in red 

 rocks ; other thin courses of porphyry are then observable, and then a 

 pebbly conglomerate, between which and the ordinary Old Red Sand- 

 stone of the parish of Lesmahago there is a considerable development 

 of porphyry, which seemed to be regularly bedded and to dip away 

 in conformity with the sandstones. 



Whilst some of this porphyry has all the aspect of having been 

 emitted contemporaneously with the sandstone and aggregated under 

 the same waters in which that formation was deposited, other bosses, 

 one of which is hornblendic, have manifestly been intruded into 

 the strata after their formation ; for both the conglomerates and 

 the sandstones are seen to be arched over such intruding rock and 

 occasionally dislocated by its protrusion. 



Conclusion. — The preceding sketch of the general relations of the 

 palaeozoic and igneous rocks of the parish of Lesmahago indicates 

 the value of the researches of Mr. Slimon, more particularly in his 

 discovery of the uppermost Silurian fossils. Having incited that 

 gentleman to send to the Museum of Practical Geology a complete 

 suite of his carboniferous fossils, all observations on the deposits of 

 that age are reserved for a future occasion. 



In regard to the Old Red Sandstone it has been shown, that its in- 

 ferior member in this district is dovetailed into the grey Silurian schists 

 and flagstones beneath it. Hence I conclude, that the greater part 

 of the red rocks exposed on the slopes of the higher grey wacke hills of 

 the parish of Lesmahago belong truly to the lower division only of 

 the system of deposits which in the North Highlands (Sutherland, 

 Caithness, and Ross) constitute the Old Red Sandstone, as there 

 composed of inferior sandstone and conglomerate, central flagstones 

 and bituminous schists, and overlying red sandstones. If my readers 

 will refer to the abstract of a communication which I made to the 

 Geological Section at the last meeting of the British Association, 

 copies of which I have addressed to this Society, they will see how 



