﻿GEOLOGY OF LESMAHAGO. 49 



fossils described by Mr. Salter 1 were found by Mr. Slitnon. These dark fossiliferous rocks, 

 the clay-slate of mineralogists, are immediately overlain 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 east-north-east or north- 

 east. 



" Old Bed Sandstone. — In the traverse along the Logan Water I did not observe any 

 unconformity between the grey beds with Crustaceans and other fossils and the overlying 

 red sandstones, the lowest courses of which are marked upon Mr. Slimon's unpublished, 

 map as ' Red Silurians.' Eor my own part, however, I would rather consider these red. 

 strata as constituting the base of the Old Red Sandstone, because they graduate up into, 

 and alternate with, the pebbly conglomerates which are largely developed near Ach Robert 

 and Waterside. 



" Some of the porphyries which are associated with the red rock in this part of the 

 series seemed to be interstratifled and of age contemporaneous with the sandstones with 

 which they dip symmetrically, and like which they are jointed and exhibit the way-boards 

 of sedimentary deposits. In mineral characters and in their interstratification with red 

 sediments, these rocks, though of much older date, present much the aspect of some of 

 the porphyries of the Rothe-todte-liegende of the Permian age in Germany. 



" The conglomerates of the Old Red of this tract differ strikinglv from those of the 

 same age in the North Highlands, where the so-called lower conglomerate is usually a very 

 coarse breccia, the huge fragments of which are more or less angular, whilst here they 

 are all worn and rounded pebbles, the largest of which scarcely ever reaches a foot in its 

 greatest diameter. 



" Most of the pebbles consist of grey and pink quartz-rock, but these are mixed with 

 other varieties of crystalline and some igneous rocks. This conglomerate zone, which is 

 fairly interstratified in red sandstone, and ranges from north to south (as laid clown on 

 Mr. Slimon's map), is much nearer to the dark grey Silurian on the Nethan River than it 

 is to the same rock on the Logan Water ; whilst on the Kype Water the two rocks are 

 still further removed from each other. Time and detailed examination will determine 

 whether this deviation of outline be due to breaks and unconformable arrangements, or 

 simply to changes in the degree of inclination of the strata. By comparing the only 

 watercourses which we examined, I am led to think that the difference of the angle 01 

 dip may sufficiently explain these diversities of superficial area, because on the Logan 

 Water we found the inclination varying from 7° to 12° only on an average ; the red beds 

 with imbedded porphyries and conglomerates, as well as the inferior grey beds, sloping off 

 to the north-east or east-north-east at these low angles, except where they rolled over bosses 

 of porphyry. On the Nethan banks, on the contrary, the beds are more highly inclined. 



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Quart. Journ. Geol. Soc.,' 1856, vol. xii, p. 26. 



