;^§ Jeffreys — Dredging among the Hebrides. 



BEITISH ASSOCIATION EEPOETS. 



SECTION C, GEOLOGY. 



I. — Eeport on Dredging among the Hebrides. 



By J. GwTN Jeffreys, F.R.S., F.G.S., etc. 



THIS exploration lasted nearly two months — namely, from the 

 24th of May to 14th July, 1866. Although its main interest 

 is of a zoological nature, yet, in some respects, it has so important a 

 geological bearing that we should be wanting in our duty to our 

 readers if we allowed it to pass by unnoticed. The following 

 extracts illustrate the advantages to be derived by geologists from 

 a careful examination of recent marine areas in investigating the 

 conditions under which the successive fossil fauna of our island 

 existed in remote periods of time. 



" Some of our most conspicuous and prized shells, that are also of 

 a northern type, are wanting in the Hebrides. Saxicava Norvegica, 

 Natica Grcenlandica, Buccinum HumpJireysianum, Buccinopsis Dalei, 

 Fusus Norvegicus, F. Tutroni, and F. Berniciensis are in this category. 

 All the above (with the exception of Buccinum HumpJireysianum, 

 which inhabits Shetland and the coasts of County Cork) are met 

 with on the Dogger Bank ; and the first two are fossil in the Clyde 

 beds. Six out of the seven being univalves, I would venture to 

 surmise that their non-existence in the western seas of Scotland may 

 have arisen from the circumstance that the diffusion of univalves is 

 slower than that of bivalves. The spawn of the former is attached 

 to the spot where it is shed, or in a few cases (eg. Capulus and 

 Calyptrcea) it is hatched within the shell of its sedentary parent ; 

 so that the fry forms a colony, and need not roam to any distance, 

 provided it has a sufficient supply of food and other requisites of 

 habitability. Not so with bivalves. These shed their ova into the 

 water, or else (as in some of the Kellia family) hatch them within 

 the folds of the mantle, whence they are excluded on arriving at 

 maturity. Their fry swim freely and rapidly by numerous en- 

 circling cilia. The metamorphic state lasts many hours. During 

 that period they can voluntarily traverse considerable distances, 

 or they may be involuntarily transported by tidal and oceanic 

 currents. Time is the only element necessary for their widest 

 dispersion over the adjacent seas, where no barrier intervenes. 

 Should, however, such an obstacle present itself, whether in the 

 shape of previously-existing dry land — like that which separates the 

 North Sea from the Atlantic — or from an upheaval and drying up of 

 the neighbouring sea-bed by geological or cosmical causes, the further 

 diffusion of any marine animals in that direction must necessarily be 

 stopped. An opposite result would, doubtless, be produced by a 

 sinking and submersion of dry land below the level of the sea, 

 whereby the diffusion of such animals would be greatly facilitated. 

 This appears to have been the fluctuating course of events since the 

 formation of the Coralline Crag, which was probably the cradle or 

 starting point of our molluscan fauna — a period long antecedent to 

 the last Glacial epoch, and incalculably far beyond the advent of man, 



