808 TTLES OORNAL OLN GE OL OGIN, 
occurrence of zeolites is very noticeable, since they are so com- 
mon in the altered modern basalts, and also occur in basalts as 
old as those of South Mountaint and of Keweenaw Point.? 
The amygdules usually stand out well from the body of the 
rock when filled with infiltrated products or when weathering has 
left them empty, giving the rock, in the first case, from a short 
distance, the appearance of porphyry, in the second, a decidedly 
scoriaceous look. 
A columnar structure so common in the modern equivalents 
of such rocks is noticeably absent. A spheroidal structure, 
however, having the same appearance as that described by Law- 
son,3 Williams* and also by Coles and Gregory,° is quite common 
in the lavas. It was in no case observed in the fragmental 
deposits. 
The breccias and tuffs—Flow breccias formed of angular 
basalt fragments imbedded in a matrix of the same material are 
found, but since they preserve the main character of the ordi- 
nary basalt flows which have just been described, they will not 
be discussed in the following passages. The pyroclastics are 
very common and quite characteristic, the characters of the beds 
being best shown on the weathered surfaces. On these the 
light grayish green fragments of varying size and more or less 
tG. H. WituiaAMs: The Volcanic Rocks of South Mountain in Pennsylvania and 
Maryland, Am. Journ. Sci., 3d Ser., Vol. LXIV., p. 491, 1892. 
2 RAPHAEL PUMPELLY: Paragenesis and Derivation of Copper and its Associates 
on Lake Superior, Am. Journ. Sci., 3d Ser., Vol. II., p. 188, 1871; also Geol. Sur. of 
Michigan, Vol. I., Part IT., pp. 19-46, 1873; Geol. of Wisconsin, Vol. III., p. 31, 1880. 
R. D. Irvine: The Copper-bearing Rocks of Lake Superior, Mon. V., U. S. 
Geol. Sur., p. 89, 1883. 
3 A.C. Lawson: Report on the Lake of the Woods, Geol. and Nat. Hist. Sur. of 
Canada (New Ser.), Vol. I., pp. 52-3, 1885. 
4G. H. WitiiAMs: The Greenstone-schist Areas of the Menominee and Mar- 
quette Regions of Michigan, Bull. U. S. Geol. Sur., No. 62, p. 166, Fig. 26, 1890. 
5G. A. J. Cote and J. W. Grecory: On the Variolitic Rocks of Mt. Genevre, 
Q. J. G.S., Vol. LXVL., p. 311, Fig. 4, 1890. 
6J. W. GREGORY: On the Variolitic Diabase of the Fichtelgebirge, Q. J. G. S., 
Vol. LXVIL., p. 48, Fig. 2, 1891. 
In these last two papers the illustrations of the structure are especially character- 
istic. 
