8 GLACIAL GEAVELS OF MAINE. 



WEATHERING. 



This is the gross result of the action of the elements on exposed rocks 

 and minerals. It is partly a chemical process, partly physical and mechan- 

 ical. The oxygen, watery vapor, carbon dioxide, nitric acid, ammonia, and 

 many other substances present in the air, either constantly or accidentally, 

 often combine chemically with the rocks or with certain of their constituent 

 minerals. Rain and snow water dissolve many minerals, usually being 

 assisted in this action by oxygen, carbonic acid, and other, gaseous sub- 

 stances absorbed from the air, from the soil, or from decaying organic 

 matter. Nor does the process stop with the simple solution of solids and 

 liquids; great chemical changes often result. The dissolved substances, 

 especially the alkaline compounds, become potent agents to effect new 

 chemical decompositions. Thus these substances are not a finality but a 

 means to an end. 



A familiar example of solution and chemical decay, and a very com- 

 mon one in Maine, is the weathering of the feldspars. By degrees the more 

 soluble alkaline silicates are dissolved and carried away, leaving an insolu- 

 ble residue, composed largely of kaolin, the characteristic ingredient of 

 clay. In like manner the pyritiferous slates and schists are readily disin- 

 tegrated. In the presence of rain water the pyrite (or marcasite) is oxi- 

 dized and hydrated so as to become ferrous sulphate, or copperas. In Maine 

 there are many places, known as "copperas ledges," where the rock contains 

 so large a proportion of pyrite that the copperas is produced in consider- 

 able quantities, and after rains in hot weather there is a strong odor of 

 sulphureted hydrogen. At the Katahdin Iron Works the chemical reac- 

 tions are still more complex; the pyritiferous slate is being rapidly decom- 

 posed, the resulting ferrous sulphate being changed to ferric hydrate by 

 organic matter. 



In addition to the insidious weakening caused by chemical decay, we 

 have the subsequent process of fracture, by both physical and mechanical 

 forces. The most common physical causes of fracture are unequal expan- 

 sion and contraction under heat and cold, and the expansion of freezing 

 water. Various forces act mechanically to produce fracture, such as move- 

 ments of the earth's crust, the pressure of overlying rock, and the impact of 

 moving bodies. The solid rock may be fractured to a limited extent by the 



