2.12 1?R0CES9)INGS OF THE GEOLOGICAL SOCIETY. [Jail. 25, 



sea ; and hence the land mnst have sunk until the Boulder-clay was 

 wholly depressed below the waters, whose temperature, as well as 

 that of the air, would, under these conditions of the land and the 

 prevalence of a cold climate, favour the development of an Arctic 

 fauna. This would flourish under and outside the rim of ice which 

 then girt the island, and would be as peculiar and as Arctic as that 

 group of Testacea which finds shelter under the ice -belt on the Green- 

 land shores. That the deposit is in no sense a drift, but a genuine 

 Arctic fauna abundant on the spot, is manifest from many facts — the . 

 distribution of the shells, their peculiar grouping, the profusion of 

 certain Arctic species, and the perfect preservation of some that are 

 tender*. All these facts render unassailable the argument for a cold- 

 climate derived from the shells. The duration of the period must 

 have been considerable to allow of such a development ; and the 

 depression must have been great, if to the height of the present shell- 

 bed we add the depth due to the species. Some of those found, as 

 Cyprina ishmdica, require a depth of 30 fathoms ; and fossils occur 

 in Arran as high as from 70 to 180 feet, making in all a depression 

 there to the amount of 360 feet. The depression may have been 

 even 100 feet more, but of this I cannot speak with confidence f. 

 Whatever may have been the precise amount of this depression, it 

 cannot have been so great as to obliterate the dominant features of 

 the land. These must have remained much the same as they are 

 now — an inner estuary sheltered by Arran, and an outer Firth ex- 

 posed to storms and swept by currents. This is clearly indicated by 

 the presence of the fine laminated clay-bed over the Boulder-clay 

 and under the shell-bed in Bute and in Clydesdale, and by the per- 

 fect preservation of the most tender shells ; while in Arran the lami- 

 nated clay is not found, and the shells are much broken. The inner 

 waters would afford conditions favourable to the deposit of beds of 

 fine sediment, which the stormy waters and tidal currents, sweeping 

 the south coast of Arran, would not allow to settle down over the 

 surface of the Boulder- clay when the land was sinking. On the 

 immigration of the fauna into these new fields after the deposit of 

 the fine, stoneless, unfossiliferous clay, the same geographical fea- 

 tures would favour a prolific development of life and the preservation 

 of the shells. The laminated clay may be due to the deposit of fine 

 sediment from the glacier-streams before the sea-bottom became 

 fit for the support of hfe — an origin suggested by Mr. Crosskey 

 in a short notice of the Chappel Hall beds, laid before the Society with 



* In the case of the Clyde beds the force of this argument is much enhanced ; 

 for there multitudes of delicate bivalves are found perfect, with tlie epidermis pre- 

 served ; northern boring species are in their natural upright positions ; and M^a 

 truncata has the siphon preserved. The fragmentary condition of many of the 

 Arran shells indicates a disturbed state of the waters on the coast where they 

 lived, and is not necessai'ily due to transport. But even on a stormy coast with 

 stony shores there are many sheltered nooks and quiet creeks, inside points, 

 behind banks and rocky ledges, where beds of unbroken shells might be expected 

 to be deposited ; and such may yet be found among the Arran beds. 



t Col, Bayly, E.E., has kindly furnished me with the heights of the banks 

 where the shell'beds occui', from the Ordnance Survey of Arran, now in progress. 



