18 Proceedings of the Boyal Physical Society. 



author added to the Pentland fossil fauna some half dozen 

 new forms of Mollusca, and greatly invigorated the study of 

 its Palasontology. 



In Mr Haswell's ste^DS followed two earnest workers, both 

 members of the Edinburgh Geological Society, Messrs D. J. 

 Brown and J. Henderson. The results of their work in the 

 Pentland Hills is summed up in a paper " On the Silurian 

 Eocks of the Pentland Hills," published in 1868,* and in it 

 we find the most complete list of Silurian organisms yet 

 given from the district in question. Brown and Henderson 

 were the first, I believe, notwithstanding Haswell's researches, 

 to systematically collect the Pentland fossils, bed by bed ; 

 and there can be no possible doubt that this method of work- 

 ing is the only true one, if accurate geological results are 

 wished for. They showed the preponderance of certain forms 

 of Mollusca in certain individual strata, thereby indicating 

 what we may call the '' zone " of such and such a species, 

 and designated by special letters. For instance, they dis- 

 covered a bed of shale weathering into cuboidal fragments, 

 and characterised by an abundance of the Brachiopod genus 

 Strophomena ; another full of a shell called Leptcena ; and a 

 third entirely peopled by our old acquaintance Orthoceras 

 Maclareni. Again, Haswell's Ehynchonellct Peiitlandica almost 

 wholly composes another stratum (Bed A), whilst a shell, we 

 shall hereafter see abounding at Lesmahagow, Trochus 

 helicites, is characteristic of the yellow bed immediately 

 underlying those red strata considered by Professor A. Geikie 

 to be of Old Pied Sandstone age. 



The results of their work in the Pentland Hills led Brown 

 and Henderson to the important conclusion that the whole 

 series represented two formations of different ages, equivalent 

 to the English Wenlock and Ludlow groups. From the base 

 of the section at the N'orth Esk (Bed A), containing Athyris 

 compressa, up to the point at which Leptcena transversalis 

 dies out (Beds E and F), the series is regarded as Wenlock, 

 whilst Bed H is considered to be of Ludlow age, including 

 certain red beds at the top of the series. By Mr Salter the 

 whole group was looked upon as Ludlow, although it must 



* Trans. Ediiib. Geol. Soc, i., pt. 1, pp. 23-33. 



