lite : its mechanical work and rock contributions. 157 



Destructive Effects. 



The destructive effects proceed either from living plants or animals, or 

 from the products of decomposition. The latter subject is briefly considered 

 under Chemical Work. 



1. The roots which come from the sprouting of a seed in the crevice of a 

 rock, as they increase in size, act like wedges, in tending to press the rock 

 apart ; and, when the roots are of large size, masses tons in weight may be 



154. 





Rocks disrupted by roots of trees, between Gloucester and Rockport, Mass. Shaler, '89. 



torn asunder ; and if on the edge of a precipice, the detached blocks may 

 be pushed off, to fall to its base. This is one of the most effective causes of 

 the destruction of rocks. Many regions of massive and jointed rocks are 

 thickly covered with huge blocks, looking like transported bowlders, which 

 are the results of this kind of upturning. The Confervse and other simple 

 plants often commence their wedging work in the smallest of rifts ; and 

 yet by constant growth cause great results. Moreover, the opening of rifts 

 and fissures gives access to moisture, and thus contributes further to rock 

 destruction by chemical processes and by frost. 



2. Boring animals, like the saxicavous Mollusks, make holes, often as 

 large as the finger, and sometimes larger, in limestone and other rocks, along 

 some seashores. Species of Saxicava, Pholas, Petricola, Lithodomus, Gastro- 

 chcena, and even some Gastropods, Barnacles, Annelids, Echini, and Sponges, 



