LIFE : ITS MECHANICAL WORK AND ROCK CONTRIBUTIONS. 155 



are distributed through the formations, and have been the source of some coaly products ; 

 but never abundantly. The trunks of Lessonia, as large as a man's thigh, lie piled in great 

 quantities on the shores of the Falkland Islands. Moreover, the growth of sea-weeds is 

 very rapid. On the coast of Scotland, and below low-tide level, "a surface chiseled 

 smooth in November, was thickly covered in the following May with ribbon kelp 2 feet 

 long, and ordinary kelp 6 feet long." But no peat-like compact beds of marine Fucoids 

 are known. Fucoids contain 74 to 80 per cent of water, some nitrogen, and are very muci- 

 laginous ; and hence ' ' when they begin a decay and become disorganized, they melt down 

 into a very small bulk, and seem almost to dissolve away." (Storer.) 



The great interest to the geologist in this subject of peat-beds is the essen- 

 tial identity between their method of origin and that of the great accumula- 

 tions of vegetable debris out of which coal-beds were made. Both were 

 accumulations of leaves and stems of terrestrial (not marine) plants, and 

 occupy, as a general thing, the region where the plants to a large extent 

 grew. The chemical processes of change were also essentially the same. 

 The burial of the ancient beds beneath thick sediments in many successions, 

 as explained on page 712, has made the chief differences. 



Protective and other Beneficial Effects. 



The protective effects of life come chiefly from vegetation. 



1. Turf protects earthy slopes from the wearing action of rills that would 

 wear a bare surface into gullies ; and even hard rocks receive protection in 

 the same way. 



2. Tufts of grass and other plants over sand-hills, as on seashores, bind 

 down the moving sands by their long creeping stems or spreading roots. 



3. Lines of vegetation along the banks of streams prevent wear during 

 freshets. When the vegetation consists of shrubs or trees, the stems and 

 trunks entangle and detain detritus and floating w^ood, and serve to increase 

 the height of the margin of the stream. 



4. Vegetation on the borders of a pond or bay serves in a similar manner 

 as a protection against the feebler wave-action. In many tropical regions, 

 plants like the mangrove, growing at the water's edge, drop new roots from 

 the branches into the shallow water. These roots act like a thicket of brush- 

 wood, to retain the floating leaves, stems, and detritus ; and, as the water shal- 

 lows, other roots are dropped farther out ; and thus they keep marching 

 outward, and subserve the double purpose of protecting and making land. 

 The coarse salt-marsh grasses along seashores perform the same kinds of 

 geological work, being very effectual agents in entangling detritus, and in 

 protecting from erosion. 



5. Patches of forest-trees, on the declivities in Alpine valleys, serve to 

 turn the course of the descending avalanche, and entangle snows that, but for 

 the presence of the trees, would only add to its extent. Such groves are 

 usually guarded from destruction with great care. 



6. Forests retard the melting of snow and ice in spring, and thus lessen the 

 devastations of floods. 



