BASIC VOLCANICS OF HEMLOCK FORMATION. 95 



Movements of the magma are sliown by a flowage structure in the 

 matrix and by the fracturing of the quartz and feldspar phenocrysts and 

 separation of the pieces in both the cement and fragments. (Fig. B, PL 

 XXIII.) 



This eruptive breccia can be seen in its best development in the NW.- 

 SE. trending ridge, just west of the small lake, crossed by the Chicago, 

 Milv^aukee and St. Paul track in sec. 32, T. 44 N., R 32 W. 



BASIC VOLCAISriCS. 



The basic volcanics are considered under the main headings of lavas, 

 pyroclastics, and Bone Lake crystalline schists. 



BASIC LAVAS. 

 GENERAL CHAKACTEKS. 



The basic lavas are so very characteristically developed that no one 

 coiild for a moment doubt their true nature, even upon the most superficial 

 examination. One of the nearly general characters is the presence of a 

 well-marked amygdaloidal texture. (Figs. A, B, Pis. XXV and XXVI, 

 and fig. A, PL XXVII.) Some of the lavas are so full of amygdules that 

 they may be correctly said to have been scoriaceous. The amygdaloidal 

 portions of the rock masses — which may be considered the sui-face parts — 

 grade over into other portions, the interiors of the lava flows, which are, 

 macrosco]Dically at least, nonamygdaloidal. Owing to the homogeneous 

 character of the basic magmas, a fluxion structure is rarely shown macro- 

 scopically, though microscopically it may be more or less well developed. 

 Columnar jointing was nowhere observed. An ellipsoidal parting, on the 

 other hand, is common. 



NOMENCLATURE . 



In a preliminary article on the Hemlock volcanics,^ I made a brief 

 mention of the occun-ence on the Upper Peninsula of Michigan of the 

 basic pre-Tertiary equivalents of the post-Tertiary and Recent family of 

 basalts. Following the Danas, Wadsworth, Williams, Iddings, Kemp, 

 Darton, and Diller, some of the most influential of the men who, in the 



' The volcanics of the Michigamme district of Michigan, by J. Morgan Clements: Jour. Geol., 

 Vol. Ill, 1895,. pp. 801-822. 



