Vol. 51.] AND INTERGLACIAL DEPOSITS IN SWITZERLAND. 375 



in stagnant pools and under climatic conditions very like those of 

 our own day. Such peat-mosses of post- Glacial age exist in abund- 

 ance in the same locality, and always afford evidence of underlying 

 boulder-clay, whose impermeable nature is eminently favourable to 

 their formation. 



As is well known, peat is composed chiefly of the dead parts of 

 weeds and moss, whose capillary structure promotes their growth 

 by the constant absorption of water from below. The pressure of 

 sand, gravel, and moraine subsequently deposited on the Pleistocene 

 Uznach, Diirnten, and Moerschwil peat-mosses, and the stoppage of 

 their further growth by the exclusion of feed-water, converted them 

 gradually into more or less slaty lignite. The quality of the lignite 

 improves with the depth — that is, with the pressure to which the 

 lignite is subjected, as is evidenced by the three superposed Uznach 

 seams, of which the lowest was of a much better quality than the 

 now remaining uppermost band. 



According to Heer, the formation of the Diirnten lignite-deposit, 

 about 20 acres in superficial area and 3*75 feet in depth, required 

 2400 years, 1 which he considered a fair estimate of the duration of 

 that interglacial period. But the three superposed Uznach lignite- 

 seams, which are together 10 feet in depth, must, on the same 

 computation, have taken 6600 years to form, irrespective of their 

 considerably larger area, and of the time required for the deposition 

 of the fluviatile sand and gravel by which they are covered. This 

 shows that Heer's figure, although it is still regarded as an axiom 

 by many geologists, can, at the most, apply only to an isolated local 

 instance, and therefore possesses no general value as to the duration 

 of the second interglacial period. 



III. Deposits near the Lake of Zug. 



Lorze Valley NagelfluJi. (Figs. 1, 2, and 7.) One of the most ex- 

 tensive fluvio-glacial deposits in Switzerland is that exposed in the 

 Lorze Valley above Baar, about 4 miles from the town and Lake of 

 Zug (8, fig. 2, p. 370). The successive layers following upon the 

 Miocene Molasse are : — (1) about 30 feet of old moraine ; (2) about 

 400 feet of fluviatile conglomerate, occasionally stratified and inter- 

 mingled with dykes and layers of sand in places hardened to sand- 

 stone ; and (3) about 130 feet of younger moraine. Longitudinally, 

 the deposit forms the so-called Menzingen plateau and covers a 

 superficial area 3 miles in width and 4 miles in length, the surface 

 being characterized by numerous hollows and depressions which 



lation of ' boulder-clay.' It is frequently confounded with the term ' ground- 

 moraine, as distinguished from surface-moraine. But of ground-moraine— that 

 is, of morainic deposit below glaciers — there is very little in Switzerland, the bulk 

 of the glacial deposits being composed of surface-moraine ; that is, of material 

 which glaciers carry on their backs and which falls to the ground when the 

 glaciers melt. 



1 Heer's computation may be shortly expressed as follows:— One acre of 

 peaty soil absorbs 15 cwt. of carbon per annum. One acre of lignite 3 - 75 feet 



in depth contains 36,000 cwt. of carbon ; hence ~:°- = 2400 years. 



