1876.] 217 [Bouv<5. 



January 19, 1876. 



The President, Mr. T. T. Bouve, in the chair. Fifty per- 

 sons present. 



The President exhibited a fine series of cut and polished 

 Porphyries from the vicinity of Boston, and read the follow- 

 ing paper : — 



On the Origin of Porphyry. By Thomas T. Bouve\ 



My object in obtaining and in bringing together the specimens 

 before me, has not been alone to show how rich our neighborhood is 

 in rocks that may prove to be of great economic value in the indus- 

 tries of the future, but also to express some views upon their origin, 

 which I have reason to believe will not receive the "assent of 

 all the geologists who have made them a study. My remarks will 

 apply not only to the true Felsite Porphyries, such as have a compact 

 feldspar base with included crystals of feldspar, but also to such as 

 are generally of like composition and character, but do not contain 

 imbedded crystals, or, if they do, the crystals are very obscure. All 

 these rocks, the true porphyries and the other felsites, vary consid- 

 erably in composition as well as in appearance, some containing a 

 much larger per centage of silex than others; but essentially they 

 are of the same general character, and all or nearly all found in our 

 vicinity have undoubtedly the same origin. 



Until within a comparatively recent period, all porphyries and all 

 such rocks as I have referred to were regarded as of igneous eruptive 

 character, and some of the text books now in use, as for instance Van 

 Cotta's " Rocks Classified and Described " in the translated edition 

 of 1866, include them among the Igneous Plutonic rocks, and no 

 idea is expressed that any of them may be of metamorphic character. 

 Hitchcock, however, in treating of the lithological character of the 

 felsites of our State, in his great work on the Geology of Massachu- 

 setts, published more than thirty years ago, says, "It seems to me 

 that in the present state of geological science, one may take it for 

 granted that compact feldspar has been once melted, but what was 

 the original rock from which it was produced ? " In saying this he 

 clearly did not mean that like lava, it was melted far beneath the 

 present surface and brought to it by eruptive action, but that it was 

 a rock derived from another by metamorphic action on the surface, 



