THE DRIFTS OF WESTERN ENGLAND

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the Irish Sea sufficient to press it in over the low grounds ofLancashire and Cheshire and against the hills which bound them.Such an ice-stream flowing across the bed of the Irish Sea wouldpick up the shells not only of the Boreal species, which had butrecently died, but also of southern species belonging to a pre-ceding age, and embedded in the loose pleiocene deposits lying onthe sea bottom. These shells being mingled in the muddyground moraine might be transported to considerable distances,a.s it is admitted on all hands that the shells in the Caithness tillwere. They would be subjected to great pressure, and henceonly those shells would survive in considerable numbers whichwere of a form calculated to withstand pressure. Gasteropodshells would contain the original matrix in many cases, and sowould the crypts in bored stones. Being merely erratics of aparticular kind they would he scratched as other erratics were.

The Irish Sea in pre-glacial times was probably no deeper thanat present, perhaps not so deep, for the land stood higher thenthan now, and therefore its fauna would be a shallow-water one.

The so-called ‘ middle sands and gravels ’ I regard asmerely the coarser constituents of boulder clay graded out by theaction of subglacial streams; hence the shells are as it were con-centrated in the sands and gravels by the removal of the finermaterials, and hence, also, the fact that the microzoa have beenremoved from their interiors by rolling over and over, and arenot found in any abundance in the sands which have been sortedby too swift a current to permit such light objects to lodge. I mayparenthetically remark that the frequent occurrence of coaly seamsin the sands is clearly attributable to ‘grading’ in accordancewith the very low specific gravity of coal, and the general ab-sence of large boulders from these deposits has reference to thedifficulty of transporting large masses.

The distribution of the shell-bearing drift seems to me to offerevidence of the most convincing character of the truth of thetheory here brought forward.

It is a very significant fact that those districts which are onthe lee side of ice-centres relatively to the great Irish Sea glacier have yielded no shelly drift even at low levels.

This is strikingly shown by the researches of Mr. J. W. E.David, who has published a description of the glacial phenomenaof South Brecknockshire and East Glamorganshire . 1 The author

1 Quart. lour. Gcol. Soc. xxxix.