424 THE AMERICAN NATURALIST. [Vor. XXXV. 
intrusions of basic igneous magmas took place before the meta- 
morphism of the Algonkian sediments. 
The author does not agree with Van Hise in ascribing the crys- 
talline character of the schists near Deadwood to the agency of 
intrusives. He regards the metamorphism as “dynamic” rather 
than “ contact." 
Isle Royale and Keweenaw Point Volcanics. — In Vol. VI of the 
Michigan Survey, Lane? and Hubbard? give a great many interesting 
details concerning the petrography of the Keweenawan eruptives. 
One of the most interesting features of Lane's paper is his discus- 
sion of the cause of the variation in coarseness of grain in rocks, and 
the application of his conclusions to the problem of the nature of the 
Isle Royale and other rock-sheets. From the fact that the Isle 
Royale sheets are characterized by an increase in the size of grain 
to their centers, he concludes that they are surface flows or lavas. 
On the other hand, he concludes that Lawson's view as to the intru- 
sive character of the diabase sheets in the Huronian beds of the 
north shore of Lake Superior is confirmed by the fact that they are 
characterized by a rapid variation in size of grain for the first few 
feet from their contacts with the surrounding rocks, and then by a 
central coarser belt of tolerably uniform grain. 
Another interesting chapter in the report is that on the differences 
in structure between small intrusive basic masses and their corre- 
sponding effusive forms. To the already recognized distinctions 
between these two forms of igneous rocks, Lane adds that the 
miarolitic cavities in intrusive masses naturally become filled with 
other minerals than the zeolitic and chloritic ones characterizing the 
corresponding pores in effusive rocks. Among the most important 
of these minerals is quartz, which often appears in diabase dikes a$ 
micropegmatitic intergrowths in the central portions of their masses. 
The microscopic and chemical features of the Isle Royale lavas are 
described in some detail. 
The report by Hubbard deals mainly with the structural problems 
presented by the interbedded lavas and sandstones in Keweenaw 
Point. It contains a few notes in the petrography of the va various 
lava beds. 
1 Ann. New York Acad. Sci. s vol xh. 
? Lane, A. C. Geological Report on pos dons Michigan. Geol. Survey d 
Midguo, Ms Me pt. i. ]sites 
* Hubbard, L. L. Keweenaw Point, with Particular Reference to the Fe 
and their Feeds Rocks, iéid., vol. vi, pt. ii 
