16 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [No7. 21, 



famous only in Scottish song, where he had found the Crustacean 

 fossils, we afterwards endeavoured to obtain a general notion of the 

 relations of all the rock-masses of the district. For the better under- 

 standing of the subject I induced Mr. Shmon to prepare a rough 

 geological map, which is exhibited, and to the sides of which he has 

 annexed two sections explaining the order of the carboniferous strata 

 of two tracts in his neighbourhood (see Appendix, p. 25). I also 

 submit a rude approximation to what will in the sequel be better 

 worked out, by exhibiting the County Map of Lanark, on which 

 I have endeavoured to combine the outlines of Mr. Slimon's Map as 

 correcting those of other observers ; no one having previously in- 

 dicated any more ancient rock in this tract than the Old Red Sand- 

 stone ; the tract which I now consider to be Silurian having usually 

 heen coloured as Carboniferous*. 



I must at once apologize for the imperfections necessarily attached 

 to this slight sketch of a district of which no real map exists. As 

 soon, however, as the Engineer Corps under the able direction of 

 Colonel James shall have published the first outlines of the Ordnance 

 Survey, I can assure my associates that the Geological Surveyors will 

 vigorously set to work to determine all those relations which are now 

 briefly touched upon in an essay, which is simply intended as a tem- 

 porary frame to hold together a few materials which are of deep 

 interest in palaeozoic geology. 



General Relations of the Rocks of the Lesmahago District. — In a 

 former communication, I invited attention to the general direction of 

 the great masses of the Silurian rocks of the S. of Scotland, which 

 have been described by various authors under that name since the 

 discovery in them of many well-known Silurian fossilsf . I then sug- 

 gested, that judging from some of those organic remains, as found in 

 the environs of Girvan, there were indications, in that parallel, of an 

 ascending order from the Lower Silurian rocks (which unquestion- 

 ably form the great mass of the S. Scottish greywacke) to the Upper 

 Silurians. At the same time it was noticed, that the strike of the 

 Girvan strata would carry them nearly to the Silurian rocks of the 

 Pentland Hills S. of Edinburgh, which have the same general direc- 

 tion, i. e. nearly from N.E. to S.W. Now, if a line be drawn from 

 the rocks N. of Girvan to the northern face of the Pentland Hills, it 

 is seen to pass over an intervening tract, throughout which basins of 

 carboniferous rocks, surrounded by girdles of old red sandstone and 

 diversified by a great abundance of igneous rocks (porphyry, green- 

 stone, &c.), are represented in all the published geological maps. 

 The discovery made by Mr. Slimon of fossils which prove to be of 

 Upper Silurian age, over a considerable area in the extensive parish of 

 Lesmahago (for this Scottish parish has a length of twenty-five miles), 

 has advanced therefore the northern frontier of the Silurian or slaty 

 rocks ; some of the localities in question being not less than twenty 

 miles to the north-west of their previously defined boundary. The 

 extent to which the Lesmahago Silurians may be hereafter shown to 



* See the maps of M'Culloch, Phillips, Knipe, &c. 

 t Quart. Journ. Geol. Soc. vol. vii. p. 137. 



