10 GLACIAL GRAVELS OF MAINE. 



TRANSPORTATION AND THE DRIFT AGENCIES. 



A vast amount of matter, held in solution by subterranean waters and 

 by surface streams, is constantly being carried off to the sea. A still larger 

 quantity is being transported in the solid condition by various other agen- 

 cies. The term "drift," as here employed, denotes solid matter which for 

 any natural cause has left its original position in the rocks, especially if it 

 has traveled a considerable distance. 



TRANSPORTATION BY LANDSLIP AND SOIL-CAP MOVEMENT. 



Greologists long ago declared that every particle that has become loos- 

 ened from its parent rock is on its way to the sea. As the result of weath- 

 ering, isolated fragments frequently become detached and fall rapidly and 

 far down steep cliffs; thus, for instance, are stones precipitated upon the 

 Alpine glaciers. Other fragments are so slowly undermined that they fall 

 only a little way at a time, or at so slow a rate that they slide rather than 

 roll down the slope. In the canyons of the Rocky Mountains, and on such 

 of the slopes of those mountains as are covered with disintegrated rock, 

 many large bowlders of stratified limestone and sandstone have slid down 

 the mountain sides many, sometimes hundi'eds, of feet. The gravel in 

 which they are partially embedded slowly weathers or is washed away, and 

 the bowlders sink with so little disturbance that the lines of stratification 

 are now nearly parallel with their original direction, although long ages 

 have elapsed since the bowlders began their journey toward the ocean. 

 Every talus or soil shows this imperceptible creep of the separate fragments, 

 and the term "soil- cap movement" has been applied to the process. The 

 simplest case is where fragments move under the action of gravity alone. 

 A more complex case arises when they also sustain the weight of other 

 solid particles, as often happens in cases of rock avalanche and landslide, 

 which in mountainous regions are important di'ift agencies. Landslides are 

 especially common during the rainy season, not only because of the lubri- 

 cating and loosening effect of water on a porous stratum, but also because 

 of the weight of the absorbed water. As is well known, extensive land- 

 slides have occurred in the White Mountains, and they are not uncommon 

 in Maine. 



At the great landslide at Goldau, in Switzerland, flashes of light were 

 seen to be emitted from the moving earth. This heat and light must have 



