i 4, 
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Ne oe ee 
132 R. D. Irving—Hornblende of the Northwestern States. 
tion to the augite.* He also shows that there are transition — 
forms between the relatively rare diorites and the diabases,’ — 
though he does not express the opinion that the former are 
altered from the latter. He also shows that the microscopic — 
characters of these greenstones indicate plainly their eruptive 
origin,” thus confirming the early views held by Foster and 
Whitney and numerous others, as against the later views of — 
Kimball, Brooks, et al. . 
In 1878 appeared Professor Pumpelly’s ‘‘ Metasomatie De- 
velopment of the Copper-Bearing Rocks of Lake Superior,” 4 
masterly account of the augitic greenstones of the Keweenawan 
series, and of the metasomatic changes they have undergone. — 
In this account the nature of the Keweenawan traps and amyg- — 
daloids was first shown, but no hornblende-bearing varieties — 
are mentioned. 
Besides Wichmann, already mentioned, several other litholo- — 
gists give descriptions in the third volume of the Geology 
Wisconsin, their work having mostly been done between 1876 
and 1878. Pumpelly, whose manuscript left him in 1878, de 
scribes a number of sections from the copper series of Northern — 
Wisconsin,” among them several uralitic gabbros,” in whi¢ 
the hornblende constituent has resulted from the change of 
diallage to augite. Pumpelly’s sections and manuscript de — 
scriptions were in my hands in 1878, and through them I first” 
ecame acquainted with this form of alteration. In PartII1of — 
this volume” I myself give briefly, the descriptions having — 
been written in 1878-79, the results of a study of some 200 — 
thin sections from the Laurentian, Huronian and Keweenawan — 
of Northern Wisconsin, among them a considerable number of 
sections of hornblendic or uralitic gabbros™ in addition to — 
those described by Pumpelly. One of the latter besides Se 
eral others are figured on the colored plates." I also describe @ 
peculiar greenstone carrying “ basaltic” hornblende and argue < 
that the hornblende in it also is secondary to augite and show 
that in all the Keweenawan greenstones carrying hornblende, — 
and then examined, that mineral is secondary to augite. 
Part III, Appendix B, A. A. Julien gives descriptions of - 
eleven rocks, among them some hornblendie greenstones from — 
the Huronian of the Penokee region, but evidence of the se@ 
ondary origin of the hornblende was not found in his sections, — 
though it is evident now that these too are uralitic. oe 
In Parts IV and VIII of the same volume Mr. ©. Ee 
Wright describes a large number of sections from the Huro — 
nian of the Penokee and Menominee regions, but although he > 
6 pp. 607, 627, 628. 7p. 624. &p. 627. 
® Proc, Am. Acad. Sci., vol. xiii, pp. 253, 309.” pp. 30-49." pp. 35, 36- 
2 pp. 53~238. 18 pp. 170, 171, 179, 180, 181, 182, 183. . 
4 Plate XVz, fig. 1, and Plate XVo, figs. 4, 5, 6. as 
te 
