748 PRINCIPLES OF STRATIGRAPHY 



Kinds of Metamorphism. 



Metamorphism is always a complex problem, but according to 

 the dominant forces three distinct types have generally been recog- 

 nized, viz., static, contactic (contagic), and dynamic. Static meta- 

 morphism is that process of alteration which goes on without the 

 interference from without of forces producing deformation, or the 

 invasion of heat from contact with a molten mass. It is essentially 

 an endogenetic process, being concerned only with those forces uni- 

 versally active within the mass. These include chemical energy, 

 gravitative rearrangement within the rock mass and the pressure 

 due to its own mass, as well as the pressure resulting from the 

 crystallization of the mass. Further, the influence of heat caused 

 by these changes within the mass — but not invading heat (or cold) 

 from without — and the influence of the omnipresent water and gases 

 within the rock mass. This complex activity is essentially metaso- 

 matic and includes in reality two quite distinct processes : the de- 

 struction of rock masses by the atmosphere and water and the 

 construction and reconstruction of rocks by the processes going on 

 within the mass. 



The process of weathering has been dealt with at length in 

 earlier chapters ; there remain for present consideration only the 

 constructive and reconstructive processes of metamorphism. To 

 the static phases of these processes of alteration the name diagenesis 

 has been applied by Johannes Walther, the name itself having been 

 used first by Giimbel in 1868 in his "Geognostische Reschreibung 

 des ostbayerischen Grenzgebirges" for the more general processes 

 of metamorphism. Andree has proposed to restrict the term to those 

 molecular and chemical transformations which the sediment under- 

 goes under the influence of the medium in which it was deposited, 

 and to which it is still subject after separation from this medium, 

 through the ordinary circulating or vadose waters, in so far as these 

 do not contain any foreign substances in solution, i. e., such sub- 

 stances as are derived from outside of the sediment. He would 

 include as chief of these changes : recrystallization, formation of 

 concretions, lithification and desalinification. In so far as this 

 restriction excludes changes due to percolating thermal waters or 

 waters bearing mineral solutions, changes which may affect locally 

 parts of various rock formations, this restriction seems warranted. 

 Such changes are to be classed under the heading of contact meta- 

 morphism and are to be placed either with thermic metamorphism 

 or are to be considered separately as hydrometamorphism. The 



