174 The Scottish Naturalist. 



very great extent. The sands probably contain about the same 

 proportion of silex, but as they become clayey they necessarily 

 contain alumina, potash, lime, or some such materials in pro- 

 portion as the clay increases. 



We have seen how of old forests flourished upon these 

 sands and pebbles, and that then were produced countless 

 tons of carbon, much of which remains still buried below 

 the upper clay beds. This carbon was not derived from the 

 sands (it was, ot course, chiefly, if not wholly, derived from 

 the atmosphere, but this is not immediately our subject), but it 

 remains an almost imperishable monument of the bygone forest. 

 In many parts of England and Scotland, forest remains are met 

 with embedded in peat, &c, in districts where trees at the pre- 

 sent time scarcely exist. I have seen hosts of tree-boles 

 (chiefly oak) exhumed from the peat of Cambridgeshire that 

 grew upon the Chalk-marl and Uppei Greensand beneath. I 

 found the peat at the foot of the " Sow of Athol " to be full of 

 birch and Scots fir, and yet from that spot not a tree was 

 visible in any direction. That last forest took its rise upon the 

 glacial material with which the valley is thickly strewn, when the 

 drainage was better than it is now, owing to the ab&ence of the 

 humus. The accumulation of carbonaceous material was pro- 

 bably the cause of the decay of the trees. The forests of the 

 Fens of Cambridgeshire and Strathearn (and what has been said 

 of Strathearn applies equally to the Tay Valley), were perhaps 

 destroyed in the same way. Nothing blocks up drainage more 

 surely than accumulating vegetable matter. The vegetable 

 mould of our fields possibly originated from forest growth, but 

 before the vegetable mould was formed the forests had to take 

 up their abode upon the gravel or sand or clay that some geolo- 

 gical phenomena had prepared. 



Scottish soils would be much more variable than they are were 

 it not for the almost universal presence of the stiff Boulder clay 

 or " till" of northern farmers. This covers a great variety of 

 rocks that would have degraded into many varieties of soil. In 

 many cases this "till" is an advantage upon what would have 

 been ; in others it may not be so. 



Ancient soils are frequently met with in the stratified rocks. 

 The Portland dirt-beds are a notable instance. These beds of 

 humus are inter-stratified with Portland limestones, so that one 

 sees forest beds resting upon ancient sea bottoms, upon which 

 the vegetables must have taken root. In these beds tree trunks 



