1862.] HARKNESS PTEIUSPIS-BEDS. 253 



The occurrence of the imprints, sometimes on the surfaces of sand- 

 rock, but more frequently on clay-beds, and that probably along 

 definite geological horizons, is suggestive of speculations as to the 

 replacement of clays by sandstones horizontally in delta- deposits ; 

 and on this subject, and its connexion with the stratigraphical cha- 

 racters of the Wealden formation, I hope to offer some observations 

 on another occasion. 



April 16, 1862. 



Thomas IPKenny Hughes, Esq., B.A., of the Geological Survey of 

 Great Britain, Jermyn Street, and Edward Petre, Esq., 38 Brook 

 Street, were elected Eellows. 



The following communications were read : — 



1. On the Position of the Pteeaspis Beds, and on the Sequence of the 

 Strata of the Old Eed Sandstone Series of South Perthshire. 

 By Professor E. Harkness, E.E.S., E.G.S. 



Introduction. — In the ' Quarterly Journal of the Geological Society,' 

 vol. xvii. p. 541, Mr. Powrie mentions the occurrence, for the first 

 time, of Pterasjns in the Old Eed Sandstone of Scotland, as developed 

 in the neighbourhood of the Bridge of Allan in Perthshire. The 

 specimens obtained by this geologist I had an opportunity of seeing 

 in his possession in the early part of this year. Being under the 

 impression that this area was occupied by that portion of the Old 

 Eed Series which is so extensively developed in Eife and Kinross, 

 and which appertains to the upper portion of the series, it occurred 

 to me that either there was something anomalous in the position of 

 these Pteraspidian remains, or that the true horizon of the strata in 

 this portion of the southern margin of the Old Eed Sandstone north 

 of the Eirths of Eorth and Clyde had yet to be determined. 



Under this impression, I was induced to examine the district 

 around the Bridge of Allan ; and I was also induced to extend my 

 observations north-westward, across the Old Eed Sandstone area of 

 this part of Scotland, to the metamorphic rocks of the southern 

 margin of the Grampians, as these occur N.E. of Callander. The 

 result of this has been to ascertain the position of the Pteraspis-beds, 

 and likewise to show a variation in this section from the lithology 

 which usually obtains in the deposits which make up the Old Eed 

 Sandstone areas lying to the N.E. of the line of this section. 



Commencing at the S.E. margin of the district under consideration, 

 we have, a little to the south of the Bridge of Allan, the great fault 

 which here separates the Carboniferous rocks of Stirlingshire on the 

 S. from the Old Eed Sandstones of Perthshire on the N. ; and along 

 this line of fault, to the W.S.W., we have that great development 

 of trap-rocks which forms the range of the Campsie Hills. To the 

 E.Js^.E. this line of fault traverses the country N. of the Ochills, and 



