1886.] Mineralogy and Petrography. 723 
swells up until it has attained eight or ten times its original bulk, 
when it assumes all the characteristics of a true pumice. Thin 
sections show under the microscope a flow structure. Devitrifi- 
cative products are abundant, while porphyritic crystals are of 
small sizes, consisting almost entirely of hornblende, magnetite 
and a brown mica. From a consideration of all the facts and a 
comparison of similar occurrences of rocks in other localities, the 
1 author concludes that the tension which affects large rock masses 
manifests itself in the structure of the nuclei of perlitic masses. 
He further observes that the parting which produces columnar 
structure in rocks owes its origin to the contraction which takes 
place during cooling. The secondary system of curved cracks 
which gives rise to perlitic and similar structures are more prob- 
ably due to the contraction which goes on as the volatile materials 
which they contain are slowly separated from them. A very im- 
portant contribution to the subject by regional metamorphism has 
recently been made by C. R. Van Hise,! who is working with Pro- 
fessor Irving on the Archæan rock of the Northwestern States. In 
this article the author gives`in a very condensed form the result 
of his studies on the rocks of the Penokee-Geogebic iron- 
bearing series in Wisconsin and Michigan, and promises that a 
full treatment of the subject will appear later in a memoir by Pro- 
fessor Irving and himself. This formation extends in a general 
east and west direction for a distance of eighty miles through 
parts of Wisconsin and Michigan. It is younger than Kewee- 
nawan and in its upper members differs in different localities. 
The group consists principally of mica schists, black mica slates, 
little clayey matter, perhaps also with a small quantity of frag- 
mental mica and some feerite. * * * -The quantity of feldspar 
was apparently considerably greater in rocks of the western part 
l Upon the origin of the Mica Schists and Black Mica Slates of the Penokes- 
Geogebic Tron-Bearing Series. Amer. Jour. Sci., XXXI, June, 1886, p. 453- 
*Cf. Bulletin U. S. Geol. Survey, No. 8. a 
