292 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 



the pseudomorphism of silicates like feldspars and pyroxenes pre- 

 supposes <the existence of crystalline rocks, whose generation this 

 neptunist never attempts to explain, but takes his starting-point 

 from a plutonic basis." 



I then asserted that the problem to be solved in regional 

 metamorphism is the conversion of sedimentary strata, "de- 

 rived by chemical and mechanical agencies from the ocean- 

 waters and pre-existing crystalline rocks into aggregations of 

 crystalline silicates. These metamorphic rocks, once formed, 

 are liable to alteration only by local and superficial agencies, 

 and are not, like the tissues of a living organism, subject to 

 incessant transformations, the pseudomorphism of Bischof." * 



I had not, at that time, seen the essay by Delesse on Pseudo- 

 morphs, already referred to, published in 1859, in which he 

 maintained views similar to those set forth by me in 1853 and 

 1860, declaring that much of what had been regarded as 

 pseudomorphism had no other basis than the observed asso- 

 ciations of minerals, and that often "the so-called metamor- 

 phism finds its natural explanation in envelopment." These 

 views he ably and ingeniously defended by a careful discussion 

 of the whole range of facts belonging to the history of the 

 subject. 



My own expression of opinion on this question, in 1853, 

 had been adversely criticised, and I had been charged with a 

 want of comprehension of the question. It was, therefore, 

 with no small pleasure, that I not only saw my views so ably 

 supported by Delesse, but read the language of Carl Friedrich 

 Naumann, who in 1861 wrote to Delesse as follows, referring 

 to his essay just noticed : 



" You have rendered a veritable service to science in restricting 

 pseudomorphs to their true limits, and separating what had been 

 erroneously united to them. As you have remarked, envelopments 

 have, for the most part, nothing in common with pseudomorphs, 

 ami it is inconceivable that they have been united by so many min- 

 ralnjriste and geologists. It appears to me, moreover, that they 

 commit an analogous error, when they regard gneisses, amphibo- 



* American Journal of Science (2), XXX. 135. 



