1855.] MURCHISON LESMAHAGO SILURIANS. 19 



courses of which are marked upon Mr. Slimon's map as " Red Silu- 

 rians." For 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 pebbly conglomerates which 

 are largely developed near Ach Robert and Waterside. 



Some of the porphyries which are associated with the red rocks in 

 this part of the series seemed to be interstratified and of age contem- 

 poraneous with the sandstones with which they dip symmetrically, 

 and like which they are jointed and exhibit the way-boards of sedi- 

 mentary deposits. In mineral characters and in their interstratifica- 

 tion 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 strikingly 

 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 sand- 

 stone and ranges from N. to S., as laid down 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 of dip may sufficiently explain these diversities 

 of superficial area ; because on the Logan Water we found the in- 

 clination varying from 7° to 1 2° only on an average ; the red beds with 

 imbedded porphyries and conglomerates as well as the inferior grey 

 beds sloping off to the N.E. or E.N.E. 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. 



In ascending order the Old Red Sandstone, including all that portion 

 of it which lies above the conglomerate and extends by Lesmahago 

 to the Clyde and Lanark, is usually of a lighter colour and freer 

 quality than the subjacent beds, and occupies a very varied outline in 

 reference to the carboniferous limestone and coal-fields on either side 

 of it and under which it is seen to dip. On the banks of the Nethan 

 Water where we examined them, the junctions are much broken, 

 and on the whole it would appear, that the older rocks have been so 

 convoluted as to form the southern edge of the great central Scottish 

 Coal-field, or the complete girdle of the Douglas coal-basin ; the S.W. 

 side of which is flanked by the Old Red of the Hawkshaw Hills, and 

 also, according to Mr. Slimon, by the Silurian rock of Bremerside Hill. 



Lower Carboniferous Rocks. — The lower edges of the Carboni- 

 ferous rocks being attained, the ascending series from them through 



c 2 



