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 GEOLOGY artificial arrangements adopted to prevent it. It is also liable to inunda- tion from fresh water flowing from the higher ground adjacent, and water discharging from more distant sources through the river valleys. To intercept and direct the former, catch-water drains are constructed. One of these, the Carr Dyke, is the old boundary between High-land and Fen-land. The Fenland consists of a variable accumulation of gravels, sands, silts, and clays, with intercalated layers of peat. The gravels, sands, and clays are mostly marine deposits, and are very irregular in disposition, but they constitute the sites of the villages. The upper peat is usually a few feet thick, only, and where it prevails at the surface the ground is uneven, and mostly devoid of villages or even hedges. The more elevated spots are principally in grass, and are called ' islands ' or ' high- lands.' From the time when the Romans first attempted to reclaim the Fens to the present day, a fairly constant struggle has been going on between man and nature for the mastery of the district. Pre-glacial and Inter-glacial Flora and Fauna No attempt has been made to give a list of the fossil plants and animals of the various geological formations otherwise dealt with ; only a few leading forms have been mentioned, and these were necessarily nearly all marine. As we approach recent times, remains of freshwater and terrestrial life increase in interest to the historian, hence the follow- ing notes. Owing probably to the shorter length of time between the Pre- glacial and Inter-glacial periods, and the more limited extension therein of complete glaciation southwards, not much variation in the animals and plants occurred, indeed it is doubtful whether any plant or animal (man excepted) occurring in this district could be quoted as conclusive evidence of the earlier or later of these two periods. Still the order of superposition of the deposits and their mode of occurrence can be used with considerable confidence in most cases. In some places in the Fens two distinct beds of peat cccur, with the lower one resting on Oxford Clay. This lower peat bed is probably a submerged forest of Pre-glacial age ; it contains remains of oak, birch, beech, hazel and yew, in the form of large prostrate trunks. The follow- ing land and freshwater mollusca were found in patches of clay in some redeposited shelly Oolitic Lincolnshire Oolite, under Boulder Clay, near to Brigstock Mill.' Succinea putris, Cochlicopa lubrtca. Pupa muscorum, Val- lonia pulchella. Helix nemoralis or Helicigona arbustorum, PisUium pusillum, also comminuted shell fragments were found in the gravel. It may be ob- served that all of these are species now living in the county, but since they Lincolnshire Oolite,' Geol. M<ig.,Aec?iAt iv., vol. ii. No. 371, May, 1895 ; see also Proc. Geol. Assoc, vol. xiv. pt. iii. (July, 1895). 35
 * Beeby Thompson, ' Peculiar Occurrence of Land and Freshwater Shells in the