METAMORPHISM -/-jz 



Age of Mctamorphic Rocks. 



Rocks of all ages may become metamorphosed, but it may be 

 stated for a restricted region that the metamorphosed rocks of that 

 region are generally older than the non-metamorphosed rocks. 

 Strictly considered, this should apply only to symphrattic rocks, 

 since later rocks may be affected by an intrusive sheet or laccolith, 

 the effect dying out upward and downward and thus not being 

 noticeable in older and younger strata. Or a set of folded strata 

 may be locally affected by intrusions which are visible in the newer 

 and not in the older strata. Again different strata are differently 

 affected by the heat of a dike which cuts all of them, and some in 

 the middle of the series may be much altered, while lower strata 

 may be less readily altered. Even symphrattic rocks are not equally 

 altered throughout, some very resistant strata being scarcely af- 

 fected by the agents which strongly metamorphose others. 



While metamorphism is undoubtedly most marked in pre-Cam- 

 bric and in early Palaeozoic rocks, it is also known in rocks of later 

 age, as shown by the metamorphic gold-bearing slates of Jurassic age 

 in the Sierra Nevada and in Sonora, and the Eocenic marble of the 

 Himalayas. See further. Correlation by Regional Metamorphism, 

 Chapter XXXII. 



BIBLIOGRAPHY XIX. 



1. ADAMS, FRANK D. 1912. An Experimental Contribution to the 



Question of the Depth of the Zone of Flow in the Earth's Crust. Journal 

 of Geology, Vol. XX, pp. 97-118. (See also King-21.) 



2. ADAMS, F. D., and COKER, ERNEST G. 1906. An Investigation into 



the Elastic Constants of Rocks, more Especially with Reference to Cubic 

 Compressibility. Carnegie Institute of Washington, Publication No. 46. 

 Ibid. American Journal of Sciences, 4th series, Vol. XXII, pp. 95-123. 



3. ANDREE, KARL. 191 1. Die Diagenese der Sedimente, ihre Bezieh- 



ungen zur Sedimentbildung und Sediment Petrographie. Geologische 

 Rundschau, Bd. II, pp. 61-74; ^^7-^3^- 

 3a. ANDREE, K. 1912. Die geologische Bedeutung des Wachstunsdruchs 

 Kristallisierender Substanzen. Geologische Rundschau, Bd. Ill, pp. 

 7-15- 



4. ARRHENIUS, SVANTE. 1912. Zur Physik der Salzlagerstatten. Medde- 



landen ivkn K. Vetenskapsakademiens Nobelinstitut, Bd. II, No. 20, pp. 

 7-25- 



5. BASCOM, FLORENCE. 1897. Aporhyolite of South Mountain, Penn- 



sylvania. Bulletin of the Geological Society of America, Vol. VIII, pp. 

 393-396. 



6. BASCOM, F. 1900. Volcanics of Neponset Valley, Massachusetts. 



Bulletin of the Geological Society of America, Vol. XI, pp. 1 15-126. 



