XOLlTHUiTBEKLAND ANJD DUllilAiT. 23 



exteusive area of laud with a gentle slope, such as Greenland, 

 produces similar effects by slowly moving down to the sea shore ; 

 and it is probable, too, that the great icebergs, some of which 

 are 4 miles in circumference, and 1800 feet high, would, when 

 grounding and moving along the sea bottom, also polish and 

 striate rock surfaces. The love of simplicity has, perhaps, led 

 theorists to attribute too much to one cause. The question 

 presented for solution by the phenomena of the boulder clay 

 is complicated ; for, as conditions in the problem, there are not 

 only glaciated surfaces and glaciated boulders mixed pell mell 

 in the lower clay, but there are similar boulders in gravel and 

 sand beds, and also stratified sands and clays, and water-worn 

 gravels ; and in addition, the occurrence of marine shells in even 

 the lower clay. A comprehensive theory must, therefore, allow 

 for the play of various agencies — the movement of ice down 

 mountains, and over extended areas of land to the sea shore, the 

 stranding of icebergs, the transport of gravel and sand by marine 

 currents, and even the quiet deposit of mud in stiller waters, dur- 

 ing the long period comprehended in the boulder clay formation. 



2. Post Plistocene. — Since the boulder clay era, there has 

 been little change in the general contour of the district : some 

 valleys have been deepened, some shallow lakes and marshes 

 drained ; and along the coast can be found traces of slight oscil- 

 lations of level. Gravel, very much rolled and rounded with 

 beds of sand, chiefly seen in river valleys, accompanied with 

 terraces on the river banks, belong to a later period than the 

 boulder clay. More recent, still, are accumulations of peat, 

 which is formed at all levels from the sea coast to the summit 

 of our highest hills, under the conditions of moisture and cold. 

 A depression of the land is indicated by peat and overthrown 

 trees (sub-marine forests) seen in some parts of the coast, below 

 the line of the lowest tides : a marked case of this character is 

 near to Howick. Frequently below the peat is a deposit of marl, 

 which, in some instances, is filled with fresh-water shells, all 

 of recent species. But besides these there have been found, 

 either in the peat or in the marl, Bofi primigenius, Bos longifrons, 



