374 MR J. GEIKTE ON THE BURIED FORESTS 



a sheet of water, portions of which are seen in the dark lochans referred to. 

 Many flows, however, do not exhibit tarns, and these, during wet seasons, when 

 the underlying reservoir has received the surplus drainage of the moors and 

 mosses, are liable to swell up and burst. It is not difficult to see how the sub- 

 jacent lake has acquired its covering of peat ; for in the gaps of this covering we 

 can watch the process of bridging-over the dark inky water in full operation. 

 Creeping out from the edges of the peat, a thick growth of Sphagnum and various 

 aquatic plants gradually encroaches upon the limits of the tarns. As this out- 

 growth becomes denser, rusty grasses begin to steal over its surface, and bright 

 tufts of Polytrichum also find there a congenial soil. So the process goes on until 

 a crust, firm enough to support such plants as cranberry, bog myrtle, and hea- 

 ther, eventually makes its appearance ; but while the upper surface of this crust 

 or cake of peat thus solidifies and thickens, its under portion rots and falls down 

 as a black vegetable sediment upon the lake bottom, where it slowly accumu- 

 lates, until in time the depression occupied by the lake may come to be filled up. 

 Many of our deeper peat mosses appear to have had such an origin. 



There are thus two kinds of peat — 1st, That which is due to the continuous 

 upgrowth from the soil of Sphagnum and its allies ; 2d, Flow-moss peat. The 

 mode of formation of a " flow" is sufficiently evident, but the origin of the other 

 peat mosses cannot always be so readily made out. It is from these last that the 

 buried trees have been dug, and hence it has commonly been thought that the fall 

 of the trunks, by obstructing the drainage, allowed moisture to collect and form 

 a marsh, in which bog-mosses sprung up. The overturning of the timber is thus 

 considered to have been the proximate cause of the formation of our peat mosses. 

 That much peat may owe its origin to such a process is certain, and several cases 

 are on record where the changes referred to have been observed in progress. But 

 this does not seem to have been the exclusive, or even the most frequent, cause 

 of its formation. There are many peat mosses in which no decayed ligneous 

 matter whatever can be detected. In the hilly districts of Southern Scotland, peat, 

 made up almost entirely of mosses, with the usual capping or crust of heather 

 peat, is of common occurrence, even on considerable hill slopes. While in the 

 majority of cases, where the peat of the higher flat-topped hills in the same region 

 is found to contain ligneous remains, the roots and branches are often of small 

 size, indicating the presence in former times of a scraggy brushwood. Now, if* 

 bog mosses could of themselves find sufficient moisture to enable them to form 

 peat on a slope of 25°, it is unnecessary to suppose that they must have awaited 

 the overthrow of mere brushwood before they could begin to grow on a flat 

 hill top. It must therefore be allowed that some peat mosses at least have 

 originated without the aid of fallen timber to collect moisture for their sup- 

 port. 



Those peat mosses, however, which exhibit the trunks of large trees, bearing 



