520 PHYSICAL GEOGRAPHY. 



The Process of Metamorpiiism. 



The alteration of the rocks is intimately connected with the elevation 

 of mountains and continents. The agents which caused the land to 

 bulge upwards also induced the conditions favorable to metamorphism. 

 Crystalline rocks abound in regions of disturbance, and are mostly want- 

 ing where subterranean influences have not been liberated through 

 dislocations and crumpling. 



The problem awaiting solution is this : How can the original hetero- 

 geneous mixtures of clay, sand, and gravel become arranged into essen- 

 tially homogeneous layers of crystalline schists of gneiss, Concord 

 granite, ferruginous and andalusite rocks.'' There is no superficial 

 resemblance between river sand and Concord granite; — what is the 

 process by which the latter can be evolved from the former.' 



In reply, we must assume the identity in mineral composition between 

 the original and derived masses, save in these few cases where evidence 

 of the withdrawal of one and the substitution of another ingredient can 

 be rendered probable. Granting this point, we have simply to show that 

 the sedimentary beds have been subjected to conditions suitable for the 

 action of elective affinities among the atoms. 



If aggregations of minerals are exposed to great heat, so as to be 

 reduced to the melted state, the conditions would be favorable for the 

 action of elective affinities, and new compounds would result. When- 

 ever this experiment is tried, the results agree with our expectations. 

 Whoever examines the slag of a furnace, or the lava freshly ejected from 

 a crater, will find many crystals scattered in geodic cavities throughout 

 the material that has been melted. The elements have been redistrib- 

 uted into new compounds, because the conditions were favorable to their 

 transposition and recombination. 



But the heat required for the melting of solids is more than sufiScient 

 for the simple metamorphism of strata. It may be that certain eruptive 

 granites have come from the absolute melting of strata ; but the schists 

 of the Atlantic gneiss have not lost their stratification. If the original 



g.v,ng credit to those who have made important suggestions towards the true explanation of the process. 

 lh,s essay must be deferred to the next volume, because the present one is already sufficiently large to be 

 conveniently handled. Extensive reference to metamorphism is also deferred for the same reason 



