328 The American Naturalist. [April, 
arranged. The author describes in detail the features of the different 
portions of the nodules and illustrates them with some handsome fig- 
ures. The central parts of the nodules are believed to be inclusions of 
gneiss that have been affected by magmatic alterations. Surrounding 
these is a zone composed of a coarse aggregate of andesine, quartz, mi- 
croline and other components of a normal granite, next a zone of a 
fine grained and radial aggregate of biotite, plagioclase, orthoclase and 
quartz, and finally the concentric shells. The zones surrounding the 
nucleus are thought to be due to contact action between the granite and 
the included gneiss. The inner concentric shell is composed of plagio- 
clase, quartz, orthoclase and biotite on the inner side and principally 
microcline on the outer. side. The outer shell is fine grained, and is 
composed of biotite, orthoclase, plagioclase and quartz. The rock sur- 
rounding the nodules possesses no special features different from those 
of normal granitites. Its material grades into that of the nodules. 
Analyses show clearly an increase in acidity from center to peri- 
phery of the nodules, which are, on the whole, more acid than the 
mother-rock. After discussing the origin of similar structural phe- 
nomena in other rocks, the author concludes that the nodules in the 
Finland granite are due to crystallization around the centers of crys- 
tallization afforded by the inclusions of gneiss. 
Volcanic Ash from the North Shore of Lake Superior. 
—N. H. Winchell and U. S. Grant’ report the existence of volcanic 
ashes in the Keweenawan series near Duluth. The rocks in question 
resemble fine grained impure sandstone. They are associated with 
diabases, basalts and rhyolites. In thin section they are found to be 
composed of fragments, apparently of vesicular lavas, in a matrix con- 
sisting of a secondary aggregate of quartz, feldspar, chlorite, epidote, 
calcite and iron oxides. A few of the fragments show traces of perlitic 
parting. 
The Diabases of Goslar.—The diabases in the middle Devon- 
ian schists of Goslar in the Harz are discussed by Rinne.* The rocks 
are always in the form of sheets interleaved with the schists. Most of 
these are intrusive, but, in a few instances, volcanic bombs and other 
evidences of the presence of surface volcanic products indicate the ex- 
istence of active voleanoes in the district, though diabase tuffs have not 
been found. From the general appearance of the upper surface of 
some of the sheets there can be no doubt but that they were lavas. 
7 Amer. Geol., Vol. XVIII, p. 211. 
® Neues Jahrb. f. Min., etc., B. B. X, p. 363. 
