1882 .] 
465 
[Merrill. 
It appeared to me that No. 270 is especially a true rhyolite with 
characteristic groundmass, predominating orthotomous feldspar, 
abundant quartz, and mica. But Zirkel has called attention to 
many holes in the section, No. 270, which were formerly probably 
filled with plagioclase which has undergone calcitic alteration and 
has suffered subsequent removal. 1 Such pores with remnants of 
calcite are well shown in No. 269. If such pores were really 
filled with plagioclase (perhaps a lime feldspar like labradorite) 
some of these cases cited, where orthotomous feldspar appeared 
to predominate over plagioclase, may be deceptive in this 
regard. 2 
Concerning Nos. 273 and 276, there would seem to me to be no 
ground for considering them anything but dacites, although, as 
has been noticed, 3 Mr. Wadsworth regards No. 276 as long ante- 
dating the Tertiary. 
Concerning No. 279 my studies lead me to agree with Mr. 
Wadsworth’s opinion that the quartz is secondary, and that this 
might be called an altered andesite. 4 
Of the trachytes of Zirkel’s Report, Mr. Wadsworth regards 
Nos. 289, 323, 324, 325, 327, 329, 330, 332 as basalts both 
macroscopically and microscopically, and excepting one or two 
which he considers allied to the andesites, as well marked basalts. 5 
No. 289 carries no olivine, some of the feldspar ledges are 
plagioclase and many are undoubted sanidins. The light-gray 
rock, trachytic rough, with its tails of ruined hornblende and the 
characteristics just mentioned, does not resemble basalt either 
macroscopically or microscopically, as it seems to me. 
Nos. 323, 324, 325, 327, 329, 330, 332 are curious rocks from 
the Elkhead Mountains. Macroscopically they generally do not 
bear the faintest shadow of a resemblance to what is called by 
most lithologists a basalt. Surely it would seem evident enough 
1 Report, p. 138; see also pp. 134, 135. 
2 Zirkel also calls attention, p. 135, to the fluid inclusions in the feldspars, a very 
exceptional occurrence in trachytic or andesitic rocks; since, in some cases it may be 
difficult to prove the orthotomous feldspar to be sanidin, it might be also difficult to 
show, on purely lithological evidence that these, or some of these, “ dacites’’ are Tertiary 
rocks. 
3 This paper, p- 464. 
4 These Proceedings, Vol. xxi, p. 266. 
5 Ibid. 
PROCEEDINGS B. S. N. H. VOL. XXI. 30 MARCH, 1883. 
