to find it brought before us in recent geological times *. Apart from such an exceptional case, I consider that, if all disturbing causes be properly taken into account, the percentage test is a good and useful guide for the chronological arrangement of the newer strata ; nor, notwithstanding its exceptional character, do I consider that a case like the one just referred to need perplex the geologist, who would seek elsewhere, in superposition or in some points of physical structure, for evidence as to place. Palaeontology is an excellent counsellor, but it should always be kept subordinate to stratigraphical geology. It indicates what may be the case, but it does not tell us what must be the case. The one has rigid, the other flexible lines ; and these lines are rarely parallel. The geologist should first determine rigorously the order of superposition, before he speculates on the distribution of the fauna. Stedfast in that mode, there need be no cause for error, however exceptional and varying the fauna may be. It is his business to determine the fact, and then, with the aid of the palaeontologist, to discover the cause and amount of variation, and to detect the principle on which the distribution of life in the period under investigation has been regulated. Palaeontology must be our guide, but not our master. It is this which gives life and interest to so many of the higher problems of palaeontological geology.
In one point of view, the geologist has the advantage over the naturalist. The latter examines the coasts and dredges in the ocean, but he can only skim the surface, whereas the former has the old sea-beds opened out to him. He can see, at any given time, what has been below the surface. The dredge may penetrate a few inches ; but the old shoals and shell-banks of the Coralline Crag sea, for example, can be opened out to the depth of 10, 20, 30 feet or more, exposing the range of life both in time and in horizontal distribution at any given epoch. What may be under the surface' of the Atlantic mud we know not. Is there a succession of strata extending down to the equivalents in time of our chalk strata ? or would the equivalent of the latter prove to be merely one part of a series, the other end of which would convey us back to Oolitic, Jurassic, Triassic, or even to Carboniferous times ? Many of the forms of life indicate a sequence in this great chain. Some of our present marine Foraminifera go back
- The absence of any known deposits in our Tertiary series of a character
like the present deep Atlantic mud is another proof that none of that part of the old ocean-bed has been raised since the Chalk period.
VOL. XXVII. f
