Barrell — Physical Effects of Contact Meiamorphism. 281 



The Rogers brothers found in 1848 that nearly all of the 

 common silicate minerals were readily attacked by water 

 carrying carbonic acid, and Richard Miiller in 187T gave 

 quantitative determinations on a number of the common 

 species. The alkalies, lime and magnesia are dissolved as 

 carbonates. The iron in the case of hornblende, epidote, etc. 

 passed into ferric oxide upon evaporation of the solutions. In 

 the action of carbonated waters upon alkaline silicates like the 

 feldspars a limited amount of silica always, goes into solution, 

 presumably in the form of hydrate. Olivine is dissolved with 

 considerable rapidity, and other magnesian silicates, including 

 serpentine, are also attacked. It is found that increase of 

 pressure renders the action more energetic. Bischof states 

 that the alkaline silicates lose their alkalies readily to carbon 

 dioxide with separation of free silica. If the alkaline carbon- 

 ates subsequently meet lime silicates, a replacement is effected 

 whereby the alkalies are for the time being retained in the 

 soil, presumably as hydrous silicates, snch as the zeolites. 



Water not only acts as a conveyor of carbonic acid but in 

 itself is efficient in the process of rock decay, by entering as 

 water of crystallization into a large number of minerals of 

 more or less stability, such as the zeolites, chlorites, serpentine, 

 talc and kaolin. 



The oxygen dissolved in water serves to precipitate hydrated 

 ferric oxide from ferrous carbonate solutions. It is to be 

 noted that the greater part of the hydrous silicates are pre- 

 eminently subterranean and not sub-aerial minerals, a free 

 exposure of oxygen, water and carbonic acid eventually 

 destroying them. 



From the original igneous minerals, then, it may be stated 

 that weathering carried to its limit would produce as solids 

 quartz, kaolin, limonite, calcite and magnesite, the latter 

 combined with the calcite as dolomite ; and would cause to be 

 taken into solution all the alkalies and some of the alkaline 

 earths. But although this is the theoretical limit of weathering, 

 it is questionable how nearly it is ever reached. 



The extreme solubility of soda feldspars over the potash one 

 is well known. In fact, under normal conditions of weather- 

 ing, orthoclase is one of the most resistant of rock-making 

 minerals. It is true also that nearly all unmetamorphosed 

 sediments show some protoxides which are still existent as 

 silicates. Some idea can be gained in a general way as to the 

 extent to which decomposition accompanies disintegration, and 

 an examination of the unmetamorphosed sediments in thin 

 section aided by an analysis would determine the degree for 

 any special case. Two extremes of decomposition will be 

 briefly considered : first in situ, the minimum amount neces- 



