362 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



points. They admit the conversion of certain granites to 

 gneissoid rocks, but do not agree that dynamo-metamorphism 

 has had any part in the origin of the true Archaean gneisses 

 and schists. Credner finds it difficult to understand how such 

 a uniform succession as is presented by the fundamental 

 crystalline rocks in the ancient mountain-systems could have 

 been the product of variable and accidental processes of 

 crushing and permeation by water. 



Zirkel draws attention to the fact that the typical funda-, 

 mental rocks and even the younger schists in many districts 

 show only very slight traces of mountain-pressure, and on the 

 other hand sedimentary rocks have often suffered gigantic 

 tectonic disturbances and pressures, and yet have not been 

 much changed in their original constitution. The petrographi- 

 cal researches of Professor Salomon have during the last few 

 years attracted considerable attention. Professor Salomon has ! 

 investigated the contact phenomena associated with deep- | 

 seated eruptive rocks in the Alps, more especially in the 

 Adamello group, and has shown that different kinds of rocks 

 have throughout long distances been altered by contact meta- 

 morphism into crystalline schists. On the basis of his ob- 

 servations in the Adamello, in the Cima d'Asta, and Predazzo 

 districts, and in other parts of the Alps, Salomon has inferred , 

 that the granite-grained bosses of the Tyrol Central Alps are 

 not, as Broegger concluded, of Triassic age, but were intruded 

 in the Tertiary epoch. The magmas solidified in the form of 

 laccolites and batholites, and as the form of the intruded 

 material frequently varied in its relation to the crystalline 

 schists during its cooling and contraction, Professor Salomon 

 thinks it possible that the latest Alpine upheaval may have 

 been induced by such variations and the consequent disturb- 

 ances of crust equilibrium. 



Although the hypothesis of dynamo-metamorphism has now 

 very numerous adherents, many questions regarding the origin 

 of the fundamental and the younger schistose rocks have yet 

 to be solved before the principles of metamorphism can be 

 securely defined, and as the subject is still under discussion, it 

 is not well suited for historical interpretation. 



