444 A TREATISE ON METAMOEPHISM. 



water is frozen while confined in a deep, strong- vessel. This squirting 

 upward at the various openings produces innumerable ice pipes. Where 

 a surface sheet of ice had formed before the restraining frozen sheet of soil 

 and water together were produced the ice pipes may raise it above the 

 surface. In a similar manner frequently a sheet of the congealed surface 

 soil and vegetation separates from the frozen material below less knit together 

 with roots. This is frequently very destructive to vegetation. As the 

 solidified surface of soil containing the roots of the various plants is pushed 

 upward the roots are broken and a crop of grain may thus be completely 

 destroyed. The ice, soil, and vegetation may be raised 5 or 10 cm. from 

 the surface. Thus we have a sheet of frozen material above the ground, 

 supported by innumerable minute pillars of ice. When first seen the 

 phenomena appear almost magical. All of the above phenomena are 

 strikingly illustrated in southern United States. 



The mechanical effects of plants in disintegrating rocks is mainly 

 accomplished by means of the roots, which first penetrate the soil and later, 

 at any given place, expand in size. They thus exert very considerable 

 pressure against the surrounding material, and universally, in the case 

 of soil, and frequently, in the case of rocks, push the material aside. In 

 some instances where the roots find lodgment in a crevice of a strong rock 

 they are unable to exert sufficient pressure to widen the openings, and as 

 the roots grow they become flat. Tree roots under such conditions may 

 have become so flat that their breadth is many times their thickness. How- 

 ever, in such cases, it is very certain that the roots exerted great pressure 

 against their confining walls. The plants which first take possession of the 

 rock surface are usually of the smaller kinds. Later larger plants appear, 

 and finally, plants of the largest size may obtain a foothold. The depth to 

 which the action of the roots extends is a function of the size of the plant. 

 The plants may, perhaps, be divided for the present purposes into (a) lichens, 

 mosses, and other small plants; (b) cacti; (c) grass, grain, and vegetables; 

 and (d) shrubs and trees. 



Lichens, mosses, etc — Lichens, mosses, and other small plants may take 

 possession of the bare rock, sending their tiny rootlets into the minute joints 

 or other fractures, or even into the pores between the grains. They thus 



