212 Yan Hise and Hoskins — Pre- Camhrian Geology. 



solutions, true injection. Metasomatism may be defined as the 

 process of metamorphism by which original minerals are partly 

 or wholly altered into other minerals, or are replaced by other 

 minerals, or are recrystallized without chemical changes, or one 

 or all of these together. Metasomatism is considered from the 

 point of view (1) of the alteration of the original mineral par- 

 ticles, (2) of the mineral particles which are produced by the 

 process of alteration, and (3) the texture of the rock produced. 

 Secondary mineral particles in the metamorphic rocks which 

 show no evidence of strain are believed to have been produced 

 by metasomatic processes under static conditions. In some 

 cases the rocks have previously been subjected to dynamic con- 

 ditions. Under mashing is included the process which is 

 usually described in geological writings under the terms 

 dynamic metamorphism and shearing. The schistose structure 

 produced by mashing in the deep-seated zone of flow develops 

 in the normal planes rather than in the shearing planes. In 

 the zone of fracture schistosity may be in the shearing planes. 



The development of each of the important pre-Cambrian 

 metamorphic sedimentary rocks is described, and the develop- 

 ment of the metamorphic igneous rocks is more briefly sum- 

 marized. The metamorphic sedimentary and metamorphic 

 igneous rocks are compared, and criteria are given for discrimi- 

 nating between the two. Among others, the conclusion is 

 reached that the finely, regularly laminated homogeneous crys- 

 talline schists having only a single structure are usually of 

 igneous origin. 



Stratigraphy. — The phenomena which may be taken as evi- 

 dence of bedding in the metamorphic rocks are described. 



The method of formation and significance of basal con- 

 glomerates are considered. The phenomena which may be 

 mistaken for basal conglomerates are described. These are 

 volcanic fragmental material, erosion conglomerates derived 

 from lavas, conglomerates formed in shallow seas by storms of 

 exceptional violence, intraformational conglomerates, auto- 

 clastic rocks, and jDseudo-conglomerates produced by igneous 

 intrusion. 



The phenomena indicating unconformity are described. 

 These are (1) ordinary discordance of bedding ; (2) difference 

 in the number of orogenic movements to which the series have 

 been subjected ; (3) discordance of bedding of upper series and 

 foliation of lower ; (4) relations with eruptives ; (5) difference 

 in degree of crystallization ; (6) basal conglomerates ; (7) gen- 

 eral field relations. It is shown that all evidence of uncon- 

 formities, even when they are marked by the most distinctive 

 proofs in little disturbed areas, may be entirely obliterated by 

 orogenic movements and the attending metamorphism. As a 

 result, originally unconformable series may be thought to be 



