380

THE FOSSILIFEEOTJS DEPOSITS

it; but the higher-lying gray stratum in -which the shellsoccur had a different origin : it is simply the partially con-solidated mud of a quiet sea-bottom ; and though its groupof organisms manifest decidedly the boreal character, I can-not doubt that they lived at a time when, either from somechange in the currents of the coast, or from the elevation ofthe protecting islands outside,—an effect of a general risingof the land,—the sea was no longer an exposed one. Theyin all probability mark that later stage of the wintry periodto which the last-formed group of our local glaciers belonged,and in which our gradually-emerging country presented, ageafter age, a broader and yet broader area, won from the deep.

One period more, and I shall have completed my survey.All the shells which have hitherto been found beneath ourlatest terraces of upheaval still exist on our coasts. Theyrepresent a time, perhaps not greatly in advance of the earlierhistoric ages, when the country had begun to exist under itspresent climatal conditions. Some of these modern, shellsare, however, found to occur in very different proportions, incertain localities, from what they do now. The only speci-mens of Pholas Candidas which I have been able to procurefrom the Lower reaches of the Cromarty Frith occur in aclay-bed of the old coast period which underlies an arablefield in the Lones of Fern, a full mile from the sea. My onlyspecimens of Scrobicularia piperata from the Frith of Forthhave been derived from the brick clays behind Portobello,more than a quarter of a mile beyond the reach of the tide.My first found Scotch specimens of Thracia convexa I col-lected last year from a raised sea-bottom near North Queens-ferry. The upheaval of the land seems to have altered theconditions, in certain localities, favourable to the productionof shells such as Scrobicularia and Pholas ; and Thracia con-vexa, though it still lives in the Frith of Forth,—which fur-nished me in the course of last su m mer with two specimens,