Ch. XIL] how accounted foe. 157 



In some cases I have seen fragments of stratified ' clays and sands, 

 bent in like manner, in the middle of a great mass of till. Mr. Trim- 

 mer has suggested, in explanation of such phenomena, the intercala- 

 tion in the glacial period of large irregular masses of snow or ice 

 between layers of sand and gravel. Some of the cliffs near Behring's 

 Straits, in which the remains of elephants occur, consist of ice mixed 

 with mud and stones ; and Middendorf describes the occurrence in 

 Siberia of masses of ice, found at various depths from the surface 

 after digging through drift. We are as yet unacquainted with the 

 mode of operation by which such intermixtures of earthy matter and 

 ice are commonly produced, but we may easily conceive their occur- 

 rence in Siberia, where the rivers flow from south to north, so that 

 the thaw begins in the country where they take their rise, while in 

 the lower regions which they overflow their channels are still choked 

 up with ice and snow. In the arctic and antarctic regions, also, the 

 frozen surface of the sea at the base of lofty cliffs is sometimes seen 

 to be the receptacle first of mud and sand, washed down from the 

 land when there is a thaw, and then, when the cold returns, of dense 

 masses of snow drifted by the wind over the edge of the cliff. Ice- 

 rafts, supporting such alternations of snow and of earthy and stony 

 matter, have been seen floating from place to place in polar latitudes. 

 Whenever the intercalation of snow and ice with drift, whether strati- 

 lied or unstratified, has taken place, the melting of the ice will cause 

 such a failure of support as may give rise to flexures, and sometimes 

 to the most complicated foldings. 



But in many cases the strata may have been bent and deranged 

 by the mechanical pressure of an advancing glacier, or by the side- 

 way thrust of huge islands of ice running aground against sand- 

 banks ; in which case, the position of the beds forming the founda- 

 tion of the banks may not be at all disturbed by the shock. Mr. 

 Geikie has described examples, in the basin of the Clyde, of ex- 

 tremely contorted beds of sand and clay, which he attributes to 

 powerful pressure experienced under a glacier or mass of continental 

 ice. 



It should also be borne in mind that lateral pressure may be exert- 

 ed simply by the weight of a heavy mass of materials thrown down 

 on some adjoining area, to which pliant beds of clay and sand may ex- 

 tend. When a railway embankment is thrown across a marsh or 

 across the bed of a drained lake, we frequently find that the founda- 

 tion, consisting of peat and shell-marl, or of quicksand and mud, gives 

 way, and sinks as fast as the embankment is raised at the top. At 

 the same time, there is often seen at the distance of many yards, in 

 some neighboring part of the morass, a squeezing up of pliant strata, 

 the amount of upheaval depending on the volume and weight of 

 materials heaped upon the embankment. In 1852 I saw a remarka- 

 ble instance of such a downward and lateral pressure, in the surburbs 

 of Boston (TJ. S.), near the South Cove. With a view of converting 



