KINDS OF METAMORPHISM 749 



name contact metamorphism is applied here to the reconstruction of 

 rocks due to contact with ( i ) heated masses such as igneous bosses, 

 dikes, sills, etc. (thermic nietamorphism or pyrometamorphism) ; (2) 

 mineralizing waters, or the alteration produced in the rock walls by 

 the rising of heated or acidulated Waters through fissures (Jiydro- 

 metamorphisni). Since such fissures are generally closed shortly 

 after by the formation of veins, no pronounced or extended altera- 

 tion is produced in the wall rock. Finally, (3) vapors bearing or 

 acting as mineralizers in passing through rock masses also produce 

 profound local changes which would come under the same general 

 heading of contact metamorphism (attiwuietainorphisui). If the 

 rock undergoes any change in character, when in contact with a 

 glacier or ice cover, this too is to be classed as thermometamor- 

 phism, while contact with a heated meteoric body would also belong 

 here. In all cases this is, therefore, to be regarded as a contact 

 metamorphism. Grabau (t6) has suggested for this process the 

 name cethoballism, from aT66^, fire, and ySaXAw, to strike, which 

 though emphasizing the heat element may. nevertheless, be expanded 

 to cover all contact cases. Rocks altered in this manner "would be 

 designated sethoballic rocks. The essential of such changes is that 

 they are local. 



Dynamic metamorphism is a term restricted to the reconstruction 

 processes initiated by tectonic movements, such as faulting, thrust- 

 ing, gliding or folding of rocks. Since it generally affects extensive 

 regions, it is also called regional metamorphism, as contrasted with 

 the local contact metamorphism. The essential causes here are the 

 pressure due to the tectonic movements, and the motion accompany- 

 ing them, the result being a crushing, shearing and rearrangement 

 of the component particles of rock, and often a recrystallization. A 

 secondary factor is the heat developed during this movement. The 

 alterations due to impact with meteoric bodies, whether hot or cold, 

 must also be classed here, and it is not improbable that in the past 

 they have played a considerable role. Finally, it must be consid- 

 ered whether the normal changes supposed to go on in the zone of 

 flowage, under the immense pressure of the superincumbent rocks, 

 do not in reality belong under the division of dynamic metamor- 

 phism. Grabau (16) has proposed the term svmpJirattism from 

 <TVfjLcf>pdTT(a, to press together, for this type of metamorphism, and 

 for the rocks of this type he suggests the term symphrattic rocks. 



As has been repeatedly emphasized, especially by Johannes Wal- 

 ther, metamorphic rocks of all kinds are naturally classed with the 

 rocks from which they are derived, and not as a separate class. 



