504 A TREATISE ON METAMORPHISM. 



regions decomposition frequently outruns transportation, and a thick layer 

 of decomposed material accumulates. Finally, there may be produced a 

 thick, nearly permanent, belt of weathered material which is protected 

 from mechanical transportation by vegetation and protects the rocks below 

 from decomposition. Such appear to be the conditions in Nicaragua along 

 portions of the proposed route of the ship canal. Hayes says that in parts 

 of Nicaragua, notwithstanding the complete decomposition of the rocks 

 and the abundant rains, the streams are clear." The process of weathering 

 must therefore be very slow, being almost wholly limited to solution. 



Regions remote from the sea. — In so far as the material is remote from the sea it 

 is long exposed to the forces of weathering before it is finally floated or 

 deposited in the sedimentary rocks, and thus in proportion as the material 

 is transported far before permanently reaching the water it is likely to be 

 decomposed. 



General statements. — While in the previous pages an attempt has been made 

 to consider separately humid regions, regions of low latitude, regions of 

 moderate topographic relief, regions of abundant plants and animals, and 

 regions remote from the sea, in speaking- of one it has been necessary to some 

 extent to consider one or more of the others. In regions which combine 

 several of these factors decomposition is at a maximum. It is only when 

 the materials of the belt of weathering in regions of high latitude, high 

 altitude, and aridity, in which life is sparse, are compared with the materials 

 of the belt of weathering in humid regions, regions of low latitude, low 

 altitude, and abundant life, that the efficiency of the latter combination of 

 conditions in the decomposition of rocks can be fully appreciated. It has 

 been seen that where the former set of conditions obtain, weathering con- 

 sists in rapid change of temperature, change of water to ice, and the work 

 of glaciers splitting or grinding the rocks into fragments; but that the 

 particles in chemical and mineral composition are substantially the same as 

 the original rock, although, of course, decomposition does always take place 

 to some extent. But in warm humid regions, in which plant life and animal 

 life are abundant, the solid rocks are not only disintegrated, but the material 

 is rapidly decomposed. This decomposition may extend from a few meters 

 to a depth of 90 meters in the subtropical and tropical regions. A result 

 of high humidity and high temperature are the wonderfully luxuriant 



a Hayes, C. W., Report of the Nicaragua Canal Commission, Appendix II, Geologic Report, 

 Baltimore, 1899, p. 112. 



