My dear M^r Darwin

Upon my return from Yorkshire I found your letter of the 18^th. and glad to see your handwriting again.² With regard to the disintegration of rocks under a coating or bed of turf, I imagine it would depend very greatly on the nature of the rocks themselves— some rocks such as the more calcareous (or marley) ones or sandstones cemented together by calcareous matter would I believe decompose and break up more rapidly under turf by the dissolving action of the vegetable acids contained in it as also of the water which would most probably be under these circumstances, more highly charged with carbonic acid—³ Other rocks however such as the hard gneiss quartzites and granitic ones would I think be protected by the bed of turf—for their disintegration goes on very slowly unless assisted by external weathering agencies—particularly frost— The organic acids or carbonic acid have little or no action on quartzites or highly silicated rocks—but attack the felspars containing lime somewhat energetically— When the rocks contain iron this metal would be rapidly taken up by water charged with organic acids or with carbonic acid or more strongly by both combined as the organic acids would first reduce the red oxide into pentoxide thus discharge the colour & bring the iron into a more soluble condition⁴

I do not however believe in the disintegrating action of the potash or soda contained in plants or in the ammonia from Nitrogenous matter— the corrosion of bad glass in stable windows is due to the action of the CO+²⁺ which is in great excess in the atmosphere such places rather than to any effect of the ammonia— The effect of water when frozen in the pores of rocks is to my mind one of the greatest agents in breaking them up not only into large fragments but also in reducing these again ultimately to sand and next to this the action of carbonic acid dissolved in water in decomposing the felspars dissolving out carbonate of lime & alcali & extracting iron—both of which effects would probably be considerably increased by the presence of organic acids in solution such as would be extracted from peaty matter—

I fear I can but add but little to the information you must yourself possess on this subject from your long & accurate observations on Nature— but it is always a pleasure to me to try and be of any slight assistance to you & I remain yours most sincerely | David Forbes