1865,] CROSSKEY CHAPPEL HALL SHELL-BED, 219 



4, On the Tellina calcakea Bed at Chappel Hall, near Airdeie. 

 By the Eev. Henry W. Crosskey. 



[Communicated by Dr. J. Bryce, M.A., F.G.S., &c,] 



One of the most perplexing cases in Scotland, upon any theory of 

 the formation of Boulder-clay, has been the alleged occurrence at 

 Chappel Hall, near Airdrie, of a bed of clay containing Tellina 

 calcarea, intercalated between two masses of true Boulder-clay. 



The facts relating to the discovery of these shells have been re- 

 corded by Mr. Smith in a paper read before the Geological Society, 

 April 24th, 1850. The present paper will simply examine the 

 question whether the superincumbent matter was, without doubt, 

 the true TiU. 



By the true Till is meant that formation covering so large a part 

 of Scotland, possessed of the following characteristics, and to which 

 it is proposed that the term " Boulder-clay" should be entirely re- 

 stricted : — 



(1) The clay contains a large proportion of striated stones, even 

 pieces of soft shale having been so firmly and yet gently held that 

 they have been polished and striated without being broken, 



(2) The stones on the whole are not far travelled, a smaU propor- 

 tion being traceable to the more distant mountains, and the greater 

 part to the flanks of the nearer ridges. 



(3) There is no stratification, although there are occasional patches 

 of sand. 



(4) The colour as well as the general character of the enclosed 

 stones is determined by the mineral character of the district. 



(5) It is closely compact, and hard to be worked, even with 

 the axe. 



The true Boulder-clay thus appears to represent a force extend- 

 ing from a distance, and yet acting with local intensity, having a 

 grasp upon the more remote heights but exercising in each imme- 

 diate place of deposition its greater strength, with a heavy pressure 

 compacting the mass of rude material, but acting with slowness and 

 persistency rather than with spasmodic and sudden efforts. 



Upon this Boulder-clay rests, through the Clyde district, a bed of 

 finely laminated clay, probably formed from the fine mud always 

 borne along by rivers issuing from ice-regions, at the period when 

 the land was sinking and the Boulder-clay had not become a sea- 

 bottom. 



The well-known shell-bed of the Glacial epoch rests upon this 

 laminated clay, in every normal section on the west coast. The 

 Glacial shells in the west are never found within the Boulder-clay 

 proper ; they are invariably above it, and separated from it, not 

 merely by an epoch of time, but by a difference of condition. 



The occurrence, however, of a Tellina calcarea bed in strata in- 

 terposed between two thick beds of Boulder-clay has been described 

 at Chappel Hall, and the ascertainment of its exact character be- 

 comes a question of considerable importance in its bearings on the 

 whole geology of the district. 



