I860.] GE1KIE OLD RED SAXDSTONE. 313 



struck with the number of felspathic dykes by which the Silurian 

 strata of that region are intersected. It was my wish to ascertain, 

 if possible, the probable date of these igneous rocks ; and for this 

 purpose it became necessary to determine the exact relation between 

 the Old Red Conglomerates and Sandstones of East Lothian, and 

 the Old Red Sandstone and Upper Silurian in other parts of South 

 Scotland. I soon perceived, however, that the determination of this 

 point had a much wider range than at first sight appeared, and that 

 in truth it bore directly upon the question of the true classification 

 of the Old Red Sandstone. The district which promised to afford 

 the most satisfactory results was the Parish of Lesmahago, where, 

 according to the section by Sir Roderick Murchjson*, an ascending 

 series could be made out from the Upper Silurian, through the Old 

 Red Sandstone, into the Carboniferous group. I visited Lesmahago, 

 and mapped the boundary-lines of the Lower Old Red Sandstone 

 and Carboniferous rocks over an area of from 80 to 90 square 

 miles. The results of this examination, in so far as they bear on 

 the Old Red Sandstone, form the first and principal part of the 

 present communication. I shall next refer to the extension of the 

 Lesmahago features to the north-cast, across the area of the Pent- 

 land Hills, into East Lothian and Berwickshire, and then point out 

 some of the bearings of the facts adduced upon the physical geology 

 of Southern Scotland during the Old Red Sandstone and Carboni- 

 ferous peiiods. 



LESMAnAGOf. 



Silurian. — The basement-rocks of the Lesmahago district consist 

 of a series of shales and flaggy sandstones belonging to the Upper 

 Silurian. They form an anticlinal axis in Nutberry Hill, whence 

 they strike south-west for six miles to beyond the village of Muir- 

 kirk in Ayrshire, when they are overlapped by Carboniferous sand- 

 stones. At Nutbcrry Hill, notwithstanding the contorted character 

 of the strata, this axis is sufficiently well-marked. Southwards, 

 however, its regularity is obscured, partly by a large intrusion of 

 felstone, partly by faulting, which possibly took place prior to the 

 deposition of the Carboniferous rocks, and partly by the way in 

 which these rocks overlap and conceal those of older date. 



Another Silurian patch occm-s in the Eaughshaw Bills as the 

 centre of another anticlinal axis. In both cases it is plain thai the 

 presenl exposure of Silurian strata in this region, so far removed 

 from the great Silurian tract to the south, arises from the Sexured 



character of tin- country, and the subsequent denudation of the 



ridges. It is interesting to observe thai the a\i> of these dei 



■ Quart. Journ. GeoL Boo. foL \ii. p. 17. 



+ The numerous notes whidfa I made of thedetailaof thegeologj of Leama* 

 !i.i\ poesibl) form toe subject of another paper. I I refer here 



id previous writers on the Lanarkshire and Ayrshire rooks, as mj presenl subject 

 i- a special one, which their labours haw not anticipated. 1 maj remark, bow- 

 thai the gem of the district is already sketched in Sir Roderick 



Bfurchieon'a -a. •.■met memoir on I Parish, Quart Joura G 



vol. xii. p. 15. 



