1883.J 425 [Wadsworth. 



Zirkel seems to confound the decomposition and surface weather- 

 ing of a rock with the molecular changes that take place in all 

 eruptive rocks after consolidation, with more or less rapidity, ac- 

 cording to the chemical composition, conditions to which they are 

 exposed, etc. (Z. 113.) Dr. Merrill seems to make a similar mis- 

 take. Dr. Zirkel asks of me the impossible: to find in afresh 

 rock the characters which I stated were to be found only in al- 

 tered rocks. The reverse I have often done; as, for instance, 

 finding the characters of andesites only partly obliterated in the 

 so-called propylites. I have also found the minerals said to char- 

 acterize propylite, but which I maintain are alteration products, 

 in rocks which Zirkel himself determined to be andesites. Dr. 

 Zirkel states that I demand that his work should conform with my 

 later published principles (Z. 110), but such a demand is not to 

 be found in my writings. I have not stated that Zirkel regarded 

 the hornblende of the diorites as an alteration product, as he says 

 I did, but that I [the writer] held that view. (W. 257 ; Z. 118.) 

 The pronoun in English refers to the preceding noun, unless tiie 

 context shows a different construction to be necessary. 



Also in case of the two syenites (Z. 113) which 1 said were old 

 andesites (W. 256, 257.), 1 hold that the feldspar predominat- 

 ing is plagioclase, and not orthoclase, and Dr. Merrill agrees with 

 me in one case. Zirkel observed plagioclase in both ; and it is a 

 question of judgment how much of the unstriated feldspar owes 

 its present condition to decomposition, and how much to its be- 

 ing really ^orthoclase. (M. 459.) 



The view that the peridotites were old, coarsely crystalline, 

 somewhat altered basaltic rocks allied to the gabbros, 1 held in 

 1879, and at that time the specimens in the collection under my 

 charge seemed to bear out that view. However the next year, 

 with more material at my command, I abandoned it, accepting 

 the peridotites as a distinct species ; and 1 think I have since then 

 taken sufficiently advanced ground in that direction. 1 But 

 Zirkel incorrectly states that I still hold this particular view. 



By the quartz found in the rhyolitic groundmass I refer to 

 that forming a constituent part of the groundmass, and not to 

 the glass-bearing quartzes porphyritically inclosed in it. My re- 



i Bull. Mus. Comp. Zool. vn 60-66, 183-187 ; Froc. Bost. Soc. Nat. Hist., 1881, xxi 

 195-197; Science, 1883, i, 127-130; Mem. Mus. Comp. Zool., 1884, xi. 



