314 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Jan. 18, 



is from N.E. to S.W., that is, parallel to the general strike of the 

 country. That other Silurian anticlines will be found in other parts 

 of this district, seems in the highest degree probable ; and hence we 

 may anticipate fresh harvests of organic remains from the prolonga- 

 tion of the Pterygotus-bearing shales of the Logan Water. 



Lower Old Red Sandstone. — The Silurian strata, as was clearly 

 pointed out by Sir R. I. Murchison*, graduate upwards into a per- 

 fectly conformable series of red shales, sandstones, and conglomerate- 

 bands, which pass by alternations into a higher and very thick group 

 of purplish-grey sandstones, often pebbly and conglomeratic. The 

 whole series above the highest of the Silurian shales must be many 

 thousand feet thick. That it represents the lower, and perhaps part 

 of the middle Old Red, seems to be indicated with sufficient clearness 

 by the geological horizon and the petrological aspect of the strata. 

 Through the kindness of — Brown, Esq., of Lanfine, I am in 

 possession of confirmatory evidence. He informs me that in the 

 sandstone- quarry of Lanfine, near Newmills, Ayrshire, several speci- 

 mens of CepTialaspis have been found, some of which are in his 

 cabinet. A drawing of one of these was sent me ; it is a well-pre- 

 served buckler of CepJialaspis Lyellii. The Newmills sandstones 

 form a part of the great series -which stretches eastward by Lesma- 

 hago and the Clyde, towards the confines of Peeblesshire ; and there 

 can be no doubt, therefore, that the whole belongs to the Lower Old 

 Red Sandstone. 



The Lower Old Red strata, as developed in the neighbourhood of 

 Lesmahago, present many points of interest, into which, however, 

 I do not enter at present. There is but one feature to which it is 

 necessary to advert, viz. that both Silurians and Old Red Sandstones 

 are everywhere traversed by dykes of porphyritic felstone, often of 

 considerable size. These dykes, so far as I have been able to observe, 

 never intersect the Carboniferous series. I have seen only a single 

 instance (that of the Nethan section, near Kerse) where the Carbo- 

 niferous strata are in contact with a felstone-dyke ; and there the 

 former, in place of showing any trace of metamorphism, present an 

 unaltered felspathic paste, in which are imbedded fragments of the 

 subjacent dyke. All the felspathic dykes, therefore, appear to be 

 older than the Carboniferous, and later than the Old Red rocks of 

 the district. 



Another series of dykes deserves incidental notice here. They 

 consist of greenstone, and are found traversing all the other rocks of 

 the district, igneous and sedimentary, as well as several large faults, 

 without undergoing any deflection. They preserve their course in 

 parallel lines from S.E. to N.W., across mountain and valley, at 

 nearly right angles to the general strike of the country. Of course 

 they are the latest rocks of the neighbourhood. 



The town of Lesmahago stands on a narrow isthmus of Lower 

 Old Red Sandstone, which expands westward into the bare heathy 

 uplands of Ayrshire, while to the east it swells out into the fertile 



* Loc. cit. 



