18 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 21, 



of Dunside and Middlefield, 1300 to 1500 feet high, and may even 

 range to Cairn Table on the south, the summit of which according to 

 the Trigonometrical Surveyors is 1 944 feet above the sea. Advancing 

 eastwards from the range of hills on the flank of Nutberry, in one 

 part of which Mr. Slimon discovered an Orthoceratite, and descending 

 the Logan Water to about a mile below the farmhouse of Dunside 

 you reach the uppermost band of the grey strata in which were disco- 

 vered those remarkable Crustaceans, which have been above referred to. 



The lowest portions of the Silurian rocks which fell under the 

 inspection of Prof. Ramsay and myself are those which are traversed 

 by the Nethan River as it flows from the Priest Hill and Nutberry Hill 

 to Cumberhead. Several dislocations and convolutions which are 

 seen on that line among the Silurian strata, as well as in the contiguous 

 Old Red, are well explained by the frequent protrusion of porphyry 

 (usually a red quartziferous porphyry). On the whole, however, 

 it was manifest to both Prof. Ramsay and myself, that in receding 

 from the Old Red boundary, and in ascending to the higher hills by 

 the course of the Nethan, we made a gathering, descending section — 

 because the strata succeeding to each other with a prevalent dip to the 

 N.E. or E.N.E. consisted successively of differently constituted mate- 

 rials. Thus, whilst the uppermost strata were dark grey and schistose, 

 other layers of lighter colours were more siliceous and formed stone- 

 bands. These are followed by other courses of shale and schists in 

 which are nodular concretions, occasionally calcareous, in which we 

 looked in vain to find a few fossils which could have led us to suppose 

 what they might very well prove to be from mineral aspect, the repre- 

 sentatives of the Wenlock formation. It is from one of these strata 

 that Mr. Slimon procured the • Orthoceratite above alluded to, but 

 which is too imperfect to be specifically determined. With some 

 undulations and several breaks, particularly in the proximity of the 

 intrusive porphyry, all these Silurian strata are inclined towards the 

 E.N.E. and N.E. , and at angles varying from 1 2° and 1 5° to verticality 

 where they roll over in flexures. 



The inferior beds exposed in the section of the Nethan are here 

 and there mineralized, and specially so where trap-rocks, chiefly 

 green-stone, have penetrated the strata ; veins of lead-ore and much 

 sulphate of barytes being there apparent on the surface. 



The section, however, which best exhibits the relations of the 

 Silurian rocks to the Old Red Sandstone is seen on the banks of 

 Logan Water between the farms of Dunside and Ach Robert. The 

 last of the decidedly dark grey and schistose beds observable in 

 descending from the flanks of the Silurian hills (Nutberry, &c.) are 

 those in which all the fossils described by Mr. Salter (see p. 26) were 

 found by Mr. Slimon. These dark fossiliferous rocks, the clay-slate of 

 mineralogists, are immediately overlaid by and pass up into red sand- 

 stone, in which there are several alternations of more or less greyish or 

 greenish-grey bands ; the whole, like the beds in the Nethan, dipping 

 to the E.N.E. or N.E., as represented in the generalized Section, p. 1 7. 



Old Red Sandstone. — In the traverse along the Logan Water I 

 did not observe any unconformity between the grey beds with Crus- 

 taceans and other fossils and the overlying red sandstones, the lowest 



