280 Barrett — Physical Effects of Contact Metamorphism. 



greatest changes in volume and mass are carbonate rocks 

 containing sufficient silica to combine with all the bases, to the 

 complete expulsion of the carbon dioxide. 



To determine with some degree of precision the extent of 

 these metamorphic changes, it becomes necessary to know 

 quantitatively the degree to which decomposition, disintegra- 

 tion and sedimentation affect the rocks upon which they 

 operate. To that end the nature of these processes will first 

 be briefly reviewed and then the completeness with which 

 they are commonly carried out. 



The Decomposition of Rocks. 



All the materials of the earth's crust which exist combined 

 with carbon and hydrogen as carbonates and hydrates have 

 supposedly been formed as a result of the decomposition of 

 silicates and were once components of igneous rocks. In 

 determining the final form in which the sediments occur two 

 classes of action have been operative in separating the 

 components of the igneous rocks. First, the difference in the 

 rate of decomposition of different minerals by which the 

 alkalies and alkaline earths are taken into solution. Secondly, 

 the forces of transportation and sedimentation which in opera- 

 tion effect a more or less complete separation, gathering 

 together the quartz as beds of sand and the particles of clay as 

 mud banks. Since this discussion is confined to general condi- 

 tions and roughly quantitative results, it is only necessary to 

 consider the common rock-making minerals of the igneous 

 rocks, the source of all the materials of the stratified series, 

 though several cycles of erosion, sorting and sedimentation 

 may have intervened between the igneous origin and the 

 existing sediments. 



The minerals necessary to consider are quartz, alkali feld- 

 spars, lime-soda feldspars, micas, hornblende and pyroxene, 

 and the disseminated iron ores. Under the conditions of heat 

 and pressure attending their formation they are naturally the 

 most stable minerals. With the lowering of the temperature, 

 however, and especially in the presence of meteoric waters 

 bearing carbonic acid, other products possess maximum sta- 

 bility and the original minerals are susceptible to change in 

 varying degrees. 



The study of rock decomposition was undertaken long since 

 by Daubree, Bischof, the Rogers brothers and others, and has 

 latterly been studied and supplemented by G. P. Merrill*. 

 From the articles of the latter many of the following 

 statements have been drawn. 



* Bull. Geol. Soc. America, vol. vi, p. 321, vol. vii, p. 349 ; Journal of 

 Geology, vol. iv. p. 704 and p. 850 ; Eocks, Eock Weathering and Soils 

 (The Macmillan Co.). 



