PRINCIPLES OF ROCK WEATHERING 705 



proniotini;" the decomposition of igneous rocks, while Brongniart, 

 writing with jjarticular reference to feldspathic decomposition 

 and the origin of kaolin, laid great stress on the acceleration of 

 the ordinary processes of decay through the electric currents 

 resulting from the contact of heterogeneous rock-masses. Darwin^ 

 believed the extensive decomposition observed by him in Brazil, 

 to have taken place under the sea, and before the present valleys 

 were excavated. Hartf gave it as his opinion that the decom- 

 position was clue to the action of warm rain water soaking 

 through the rock and carrying with it carbonic acid derived not 

 onlv from the air, but from the vegetation decaying in the soil 

 as well, together with organic acids, nitrate of ammonium, etc. 

 Further that the decomposition had gone on only in regions 

 once covered by forests. Heusser and Claraz,^ suggest that the 

 decomposition was brought about through the influence of nitric 

 acid. They say " it is without doubt determined by the violence 

 and frequency of the tropical rains, and by the dissolving action 

 of water, which increases with the temperature. It is necessary 

 to observe, moreover, that this water contains some nitric acid, 

 on account of the thunder storms which follow each other with 

 great regularity during many months of the year." 



Belt * in discussing the extensive decomposition observed by 

 him in Nicaragua says "This decomposition of the rocks near 

 the surface prevails in many parts of tropical America, and is 

 principally, if not always, confined to the forest regions. It has 

 been ascribed, and probably with reason, to the percolation 

 throug'h the rocks, of rain water charg^ed with a little acid, from 

 the decomposing vegetation." 



The elder Agassiz^ laid much stress on the decomposing 

 effects of hot water from rainfall, while Mills ^ attributed no 



' Geological Observations, p. 417. 



* Physical Geography and Geology of Brazil. 

 3 Ann, des Mines, 5^ Series, 17^, i860. 



* The Naturalist in Nicaragua. 1874. 

 5 Jour, in Brazil, p. 8g. 



* Am. Geologist, June 18S9, p. 351. 



