It is more difficult to understand the absence of later Quaternary shells, such as those of the Clyde beds, only three of which have been recognized among the late dredgings. Does it arise from the more littoral and shallow forms of that class being stayed by climatal conditions near our shores, while the deeper-sea forms passed on southward free from the influences which affected the others? That a great proportion of the deep-sea forms had migrated during and since that period is probable from their wide diffusion and large numbers. Mr. Jeffreys has enumerated fifty of these more recent northern Mollusca which are not known in a fossil state ; and of the Echinoderms and Crustacea mentioned by Dr. Carpenter and Prof. Wyville Thomson a large proportion are Norwegian, Spitzbergen, and other high-northern forms.
From what I have previously said, you will have understood that, lithologically, there is but little resemblance between the Atlantic mud and our typical white chalk, none that could have ever led a geologist into any error of determination. In fact, in no part of the area yet explored is there any thing at all to be identified lithologically with the true white chalk. Even if it were found that the superposition were conformable, the difference of mineral character is too marked. At the same time it is to be observed that the area of the Atlantic is so vast that, variable as the deposit now going on seems to be, it is probably little, if any, more so than that which went on in some parts of the Chalk series in the bed of the Chalk - ocean over the old European area. Of the rate of the present deposit we know nothing. Is it even going on everywhere over the deep Atlantic ?
Therefore, although I think it highly probable that some considerable portion of the deep sea-bed of the mid-Atlantic has continued submerged since the period of our Chalk, and although the more adaptable forms of life may have been transmitted in unbroken succession through this channel, the immigrations of other and more recent faunas may have so modified the old population, that the original chalk element is of no more importance than is the original British element in our own English people. As well might it have been said in the last century, that we were living in the period of the early Britons because their descendants and language still lingered in Cornwall, as that we are living in the Cretaceous period because a few Cretaceous forms still linger in the deep Atlantic. Period in geology must not be confounded with " system " or " formation." The one is only relative, the other definite. A
