ON THE COAST OF MAINE. 331 



ences in harrlness are great, lias been marked by the peculiar corrugation which, when 

 made apparent by the level of the sea. as on the coast line, comes into peculiar prominence. 



2. The wearing has been determined by the following additional circumstances. 

 (A.) The velocity of the stream. (B.) Its depth. (C.) The angle of declivity, and (D.) 

 the arming of the ice sheet at its bottom with pebbles of sufficient hardness to have a 

 strong cutting power. 



3. As an adjunct to the ice action, — the subglacial streams ; these doubtless existed then, 

 as they now exist in Gi'eenland, and must have coiiperated in many cases with considerable 

 effect in the formation of the deeper cuts of the ice. 



4. There is an important conclusion, a corollary to the preceding propositions, that the 

 deeper any cut becomes through the action of glacial movement the more rapid the wear. 

 The accumulation of the ice in the valleys tends to relieve the weight acting upon the hill 

 tops, and thus magnifies the erosion on the low grounds at the expense of the wear of the 

 higher surfaces. Thus it is seen that while ice in some of its conditions of action is indeed 

 a planing agent, it is not strictly correct to regard it as necessarily or even generally an 

 agent which acts against existing reliefs. 



So far as I have been able to see, the excavations which cut up the shore of Maine pre- 

 sent no difficulties which demand especial explanation. All the great fiord-cut shores of 

 the world are excavated in highly metamorphosed materials. It is a flict, that metamor- 

 phosed rocks, especially the series which is affiliated with granite, vary far more in compo- 

 sition than any of the unchanged sedimentary beds. In most cases this great variety of 

 structure depends upon the action of the penetrated waters charged under varied conditions 

 of heat and pressure with the chemical agents of change. Fissures which gave passage to 

 these streams have naturall}' had their borders far more completely changed than the other 

 parts of the mass. Other agents have cooperated to localize the characters of our meta- 

 morphosed rocks, making them as unlike in their horizontal sections as our unchanged 

 rocks are uniform in this direction. Chief among these is the contorting action to which 

 they have been subjected, which by bringing different beds to the surface in their narrow 

 areas, has powerfidl}- contri])uted to the diversification of the hardness of the surface. 



The evidences of great excavation along the coast of Maine are invariably associated 

 with either of the two following circumstances, the previous existence of great drainage 

 channels, obviously determined as regards position and direction Ijy running water, or the 

 presence of great obstacles which have, by partly diverting the ice stream, concentrated its 

 wear in narrower limits. The truth of the first of these propositions is evident to any one 

 who will take the principal rivers of Maine, examine their drainage basins, and observe their 

 dependence on features which could only have had a value with reference to the action of 

 fluid water. If the direction of the river systems had been in any way determined by the 

 ice action, they would have been very differently arranged from what they are now. It is 

 evident that they are the work of water acting in its fluid form, .and that the action of ice 

 has been limited to a certain amount of Ijroadening and deepening the chaiuiels it already 

 found in existence. 



The exceptions to this general rule are found in those cases where the presence of an 

 obstacle of great height has interfered with, or where the diflerence in hardness of the 

 materials flowed over has greatly modified, the cutting action of the ice. At Mount Desert, 

 for example, we have a great mass of high land extending in a general northeast and 



