Wadsworth.] 426 [October 3, 



mark was intended as a general objection to basing specific dis- 

 tinctions on slight grounds and was not intended to be confined to 

 any single section. (M. 469 ; W. 269; Z. 115.) 



Concerning the greenish alteration spots that I claimed Zirkel 

 mistook for olivine, but which he' denies, it maybe noticed that 

 Dr. Merrill observed that the hand specimen now bearing the 

 number 531 was not the one described by Zirkel — since it is a 

 fresh rock. But he fails to observe that my language implies that 

 the rock seen by me was old, altered, and much decomposed ; and, 

 therefore, probably the rock described by Zirkel, which is now 

 missing. The section and hand specimen both agreed, and the 

 collection was carefully examined by me to see if by any possible 

 chance any other rock could have been described by him. No 

 other one could be found and I must still hold that the rock I 

 saw was the one Zirkel saw, and that he made the mistake 

 pointed out ; but the specimen now seems either to have vanished 

 or is doing duty in another part of the collection. I make my 

 statements, of course, on the supposition that the collection was 

 fairly authentic at the time of my visit. (M. 469, 470; Z. 116; 

 W. 270.) 



My reasons for regarding No. 608 (2006) as a rhyolite instead 

 of a basalt (tachylyte or hyalomelane) are the following: It 

 contains feldspar crystals some of which are macroscopical, al- 

 though Zirkel states that it contains no macroscopical secretions. 

 These feldspars under the microscope prove to be sanidin. The 

 glass shows lluidal structure and is composed of an irregular mix- 

 ture of dark, light, and yellowish brown glass, filled with the com- 

 mon globules seen in rhyolitic glasses of this character. The 

 structure of this glass is similar to that imperfectly represented 

 in volume vi, plate vi, fig. 3, and closely like that in the sections 

 which in 1878 bore the numbers 558, 1444, 1474, and 1676 (vol. 

 VI, pp. 171, 185, 186, 191 ; Nos. 351, 417, 424, 450). It presents 

 the characteristics that I, as well as Zirkel, have repeatedly ob- 

 served in rhyolitic glasses; while it does not, to my mind, show 

 a single character belonging to any basaltic glass that I have ever 

 seen ; therefore, I have no reason to doubt its being a rhyolite 

 until it shall be proved by chemical analysis to be a basalt. Its 

 want of solubility is certainly consonant with its rhyolitic charac- 

 ter. (W. 271 ; M. 470.) 



