G. W. Bulman — On Underdays. 353 



■with the 'Coal-seam and its underclay. Those peat mosses, again, 

 which form the most recent layer in the filling up of lakes do not 

 require that the substratum should be universally an impervious 

 clay ; for the same causes which led to the formation of the lake 

 in the first instance will prevent the drainage of the filled up area. 



Sir Charles Lyell, indeed, in describing lake deposits in general, 

 gives shell-marl as the stratum immediately below the peat, thus : 



"If we drain a lake which has been fed by a small stream, we 

 frequently find at the bottom a series of deposits, disposed with 

 considerable regularity, one above the other ; the uppermost, perhaps, 

 may be a stratum of peat, next below a more dense and solid variety 

 of the same material ; still lower a bed of shell-marl, alternating 

 with peat or sand, and then other beds of marl, divided by layers 

 of clay " (Elements of Geology, p. 3). 



And in the Meadows, Edinburgh, formerly the Borough Loch, 

 the following section was met with in digging a drain, and is thus 

 mentioned in the Survey Memoir of the Geology of the Neighbour- 

 hood of Edinburgh : 



" A bed of lake-peat, from a few inches to upwards of a foot in 

 thickness, was found immediately below the soil. A thin lay of marl, 

 with the common freshwater shells, underlaid the peat and rested 

 on silt, sand, or fine gravel, below which came the Boulder-clay." 



In the Survey Memoir of the Geology of East Lothian two 

 similar cases of peat immediately underlaid by maid are mentioned 

 (pp. 67, 68). 



According to Prof. Heer, again, the ground for the growth of peat 

 as the final deposit in the silting up of a lake is due to Mollusca, 

 and is thus described : 



" The formation of an impermeable ground is the work of small 

 mollusca, which live in the water in great abundance. After death 

 their shells decompose, and gradually produce, with the inorganic 

 deposits of the watei", a sort of calcareous loam, called hlanc-fond 

 (white ground) in Neuchatel, and laJce-chalh in the canton of Zug " 

 (Heer's Prima3val World of Switzerland, Heywood, vol. i. p. 24). 



Clearly, then, the origin of underclays cannot be explained by the 

 causes now in operation in the filling up of lakes. 



As regards the occurrence of peat otherwise than as a lacustrine 

 deposit, some interesting details are given in the Survey Memoir of 

 the Geology of the South-west part of Lincolnshire. Several beds 

 of peat at different levels and interstratified with clay, silt, sand, 

 etc., occur. The following sections may be taken as examples; 



FEET. 



(1). Brown silty soil ... ... ... ... 1 



Reddish brown silt ... ... ... 1 to 4 



Blue clay ... ... ... ... 1 



Drab -coloured sandy clay ... ... ... 4 to 6 



Peat and peaty- clay ... ... ... 1 



Blue clay with marine shells ... ... 6 



Blue clay with peaty matter and wood at bottom 1 6 



Peat black and compact ... ... ... 1 



Coarse angular sand ... ... ... 1 



Bluish Boulder-clay, touched ... ... 1 



(Brickyard one mile N.N.W. of Spalding, p. 107.) 



DECADE III, VOL. IX. — NO. Till. 23 



