4 2 6 PREHISTORIC E UR OPE. 



and Cyclas; the carbonate of lime being separated by them from 

 the water. The shells themselves have frequently been obli- 

 terated by subsequent chemical changes, so that often the marl 

 appears to be quite destitute of any organic structure. Occa- 

 sionally the marl contains intercalated layers of loam and sand. 

 In other cases, again, we may find little or no marl, the beds 

 underlying the vegetable layers consisting of loam, sand, and 

 gravel. These detrital accumulations were no doubt washed 

 into the lake by rain and rivulets, and they show us how it was 

 gradually silted up. While this process was being carried on 

 trees grew upon the margin of the lake and often dropped their 

 leaves, fruits, and branches into the water. Now and again, 

 too, deer, oxen, and other animals, dying upon the banks of the 

 streams or the lake itself, were floated into the latter, perhaps 

 by sudden freshets, and thus their skeletons eventually became 

 entombed in the mud accumulating below. While the lake 

 diminished in extent, the woodlands of course increased, the 

 trees always occupying the flat reaches as the water retired. 

 And so, by and by, the lake disappeared, and its site was occu- 

 pied by forest. In some cases, however, a streamlet continued 

 to flow across the dried-up lake-bed, while frequently one or 

 more deep pools or lakelets remained, and have continued to the 

 present day, as in the case of Linton Loch, in Boxburghshire. 

 After the great oaks and their congeners had flourished for a 

 long time, the low-lying ground upon which they grew was 

 converted into a marsh. The trees decayed and fell to the 

 ground, and were gradually enveloped by bog-mosses in their 

 upward growth. Here and there traces of an upper forest-bed 

 sometimes occur in the peat, which would indicate that the 

 marshy conditions ceased, and the area became well fitted for 

 the growth of a second forest. The overlying peat, again, 

 proves that marshy conditions returned, and the second forest 

 succumbed in the same manner as the first. 



Eemains of the postglacial mammalia have been from time 

 to time obtained in these lacustrine deposits in every part of the 

 country. Among other forms we find Bos primigenius, B. longi- 



