254 PEOCEEBINGS OF THE GEOLOGICAL SOCIETY. [Apr. 16, 



passing through Perthshire by the Carse of Gowrie, it separates, in 

 the latter area, the lower members of the Old Eed Sandstone on the 

 North (as seen at Rossie, Bakudery, and in the neighbourhood of 

 Dundee) from the higher members on the South, as exhibited at 

 Clashbinnie. 



Old Red Sandstone Series at the Bridge of Allan. — Immediately N. 

 of this line, in South Perthshire, at the Bridge of Allan, the Old Red 

 deposits make their appearance. The lowest strata which occur here, 

 and which are seen on the road from the village by the well to Wolf's 

 Hole Quarry, on the Westerton estate, consist of conglomerates 

 made up of fragments of trap ; and these conglomerates have, as we 

 ascend in the series, deposits of grey sandstones intercalated with 

 them. 



These latter dip N.W. at an angle of about 20°, and they gra- 

 dually become so developed as to exclude the conglomerates which 

 are so abundant beneath them. 



These grey sandstones are now worked at Wolfs Hole Quarry; 

 and, as seen here, they are covered by a mass of trap. It is in this 

 quarry that the only recognized specimens of Pteraspis have been 

 found in Scotland ; and the species appears to Prof. Huxley, who 

 examined the specimens obtained by Mr. Powrie, to be P. rostratus. 

 These Pteraspidian remains are by no means uncommon here ; but 

 they are usually in an imperfect condition. Besides Pteraspis, I have 

 procured from this locality Cephalaspis ; and Mr. A. Bryson, of 

 Edinburgh, informs me that he also obtained this latter genus from 

 the same locality some years ago. 



The grey sandstones are well seen in the course of the Allan above 

 the last-mentioned locality. They form the bed of the river to beyond 

 Kippenross House, and are also seen in ascending the stream to 

 beyond Dunblane; but they gradually change their colour and 

 become purple flaggy sandstones. The north-west dip at the same 

 angle, however, prevails along the course of the Allan, from the 

 Bridge of Allan to above Dunblane. 



East of the Bridge of Allan. — In the district which lies on the east 

 side of the Allan Water the same grey sandstones occur, succeeded 

 by the purple beds. At Stonehill Quarry, a mile and a quarter E. 

 from Dunblane, the former are wrought, and these quarries are the 

 source from whence the building-stone of Dunblane is principally 

 obtained. Some of the beds are micaceous and flaggy ; and, on the 

 whole, the strata at Stonehill have a great lithological affinity to the 

 Forfarshire flags. I learn from Mr. Page that Cephalaspis Lyellii 

 has been found in this quarry. The angle of dip and the direction 

 here also conform to the strata traversed by the Allan Water. 



East from StonehiU, and flowing along the northern margin of the 

 chills, is a stream called Alt Wharry : this separates the traps on 

 the S. from the grey beds of the Old Eed Sandstone on the IS". ; and 

 the character of the vegetation on these respective rocks well marks 

 the difl'erence in their mineral nature. The former is clothed with 

 a fine green herbage, whUe on the latter brown benty grasses prevail. 

 These latter cover the surface of Sherra Muir, which, where inter- 



