654 A TREATISE ON METAMOKPHISM. 



by Shaler, as already noted (p. 127), to be 3 to 5 per cent of the superficial 

 area at certain places along the New England coast," and this corresponds 

 to an increase in volume from 0.52 to 1.12 per cent. Where one set of 

 openings which originally existed or has been produced by fracturing has 

 been closed by cementation, metasomatism, and injection, a new set of 

 ruptures may form, thus further increasing the volume of the rocks, and 

 these openings may again be closed by cementation. Of course, in such 

 rocks as were originally very porous — for instance, the sandstones and 

 amygdaloids — the alterations of the belt of cementation may take place 

 with no gross expansion of the volume of the rocks, the necessary spaces 

 being found in the original openings. 



The conditions favorable to cementation and metasomatism are usually 

 the same ; therefore the two processes naturally occur together. In some 

 cases, such as cementation of a pure quartz sand, cementation may take 

 place without marked metasomatism; but this is exceptional. Metasoma- 

 tism is almost invariably accompanied by cementation, and usually where 

 the former process is important the other is sure to be of consequence. 

 Where injections occur it has been seen that, besides closing the openings, 

 an extended and profound exomorphic effect is produced. Further, it has 

 been, seen that this is especially likely to be true when the exomorphism is 

 indirect or the work of solutions. But the alterations accomplished by the 

 solutions as an agent are cementation and metasomatism. Therefore, where 

 injection is an important metamorphosing influence, cementation and meta- 

 somatism are exceedingly active and widely extended. The underground 

 waters are heated. These heated waters dissolve a large amount of the 

 material of the injected and injecting rock; they receive a further contribu- 

 tion furnished by the solutions emanating from the injected rock. Solu- 

 tions thus enriched travel for a greater or less distance, but ultimately a 

 portion of the material is deposited, thus cementing the rocks. Just as 

 injection promotes cementation it promotes metasomatism, and for the same 

 reasons. The solutions are heated and rich in mineral materials, and there- 

 fore the mineral particles present in the injected rock are modified. It 

 follows from the foregoing that the chief indirect exomorphic effects are 

 accomplished by cementation and metasomatism. 



"Shaler, N. S., The crenitic hypothesis and mountain building: Science, vol. 11, 18S8, p. 281. 



