632 REV. THOMAS BROWN ON THE ARCTIC SHELL-CLAY 



lying the shell-clay, with that of Caithness, as investigated by Mr Peach. The 

 same thing is true of the brick-clays, and we should thus get into all kinds of 

 confusion did we attempt to make out the series by attending merely to the 

 structure of the beds. It is by examining their fossils or their stratigraphical 

 position, as compared with other fossiliferous deposits, that we shall be safe in 

 our inferences. Proceeding in this way, the whole deposits referred to in this 

 paper may be arranged in three divisions. 



I. The Elie and Errol shell-claj r , with its underlying and (in the sense 

 explained above) contemporaneous boulder-clay. All over the Forth and Tay 

 districts this is the lowest and oldest portion representing the period of the Arctic 

 cold. 



II. There is an intermediate series of deposits. In the above sections de- 

 scribed in this paper these beds are awanting, represented only by the line of un- 

 conformity formerly referred to as making a break in the series. At a single 

 leap we pass from the rigour of Arctic cold to the present climate. This chasm, 

 however, may quite well be filled up from other parts of the country where we 

 pass through a whole series, in which a less and less degree of cold is gradually 

 indicated. About the centre of this scale would seem to lie the Clyde beds, de- 

 scribed by Mr Smith of Jordanhill, having beneath them those of Aberdeen, so 

 well investigated by Mr Jameson of Ellon, and above them those of Fort William 

 and Caithness. Step by step we can trace the passing away of the Arctic cold. 

 These intermediate deposits are wholly wanting in the above sections, unless 

 it may be the portion of high-level unfossiliferous gravels (Section III.), which 

 show, as we saw, some trace of glacial cold. 



III. The group of deposits representing the present climate. 



It will not, I trust, seem presumptuous if I suggest that geologists may yet 

 find in these Elie and Errol deposits the starting-point for a more rigorous classi- 

 fication of our superficial beds throughout central Scotland. In studying the 

 geographical distribution of northern shells as they at present exist, it is well 

 known that conchologists have recognised two great provinces — the Arctic, or 

 most northern, and the Boreal, or sub-arctic, the less northern. Now, as this 

 division holds good for the conchologist in separating the two groups in regard to 

 space, so the geologist may find it hold good in separating two groups of deposits 

 in regard to time. We have in these superficial strata first an Arctic time, with 

 its own set of deposits, when the climate is that of the Polar regions, characterised 

 by shells strictly Arctic, and next a sub-arctic time, also with its special beds, 

 characterised by the group of Boreal shells. The type of the one is the Leda 

 truncata, found everywhere in the lower and colder deposit ; it represents the 

 Arctic province. The type of the other is the Tellina proxima, found everywhere 

 in the Clyde beds in countless numbers.* It is properly a Boreal shell, repre- 



* It occurs but very scantily at Elie. 



