572 The American Naturalist. [June, 
more completely metamorphosed than the members of the phyllite 
series, for in addition to andalusite there is an abundant development 
of cordierite in them, where they approach the eruptive. As 
in the case of the diorite contact belt, there have also been recog- 
nized in the belt surrounding the tonalite two distinct contact zones. 
In the outer one the normal gneiss and mica-schists have been changed 
into cordierite and andalusite varieties, containing a fibrous orthoclase. 
In the inner zone all resemblance to gneiss and mica-schists has dis- 
. appeared, and the rocks have become aggregates of cordierite, anda- 
lusite, plagioclase, sillimanite, tourmaline, garnet, spinel, corundum, 
and zircon. The replacement of the orthoclase of the outer zone by 
plagioclase in the inner one indicates a difference in the composition 
of the two belts, but this is thought not to be a result of more intense 
metamorphism, but as due rather to a difference in the composition of 
the original materials. The cordierite in these rocks is colorless and 
non-pleochroic, but it becomes brown and pleochroic on heating. It 
often twins parallel to œP, and alters readily to a pinnite-like sub- 
stance. The other constituents possess no unusual peculiarities, They 
are all well described, but none are analyzed. Dykes of porphy- 
rite cut the schists, and a single one occurs in the tonalite, but none of 
them merit special mention in this place. The most valuable con- 
tribution to the study of the crystalline schists that has appeared for 
some time is entitled ‘‘ The Greenstone-Schist Areas of the Menom- 
inee and Marquette Regions of Michigan.” In it the author, Dr. 
G. H. Williams,’ discusses the origin of the crystalline schists found so 
widely spread over the country bordering on Lake Superior. The _ 
paper opens with an excellent historical review of the recent work on 
crystalline schists in Europe, in which all the important articles on 
dynamically formed schists are abstracted. This chapter, together 
with the foot-notes accompanying it, form a splendid resumé of the 
_ present state of our knowledge on this subject, and is well worthy the 
study of all geologists, particularly of those who still hold to the — 
_ belief that all schistose rocks were originally laid down as sediments. 
In the special part of the volume the author describes the present fea- 
tures of the green schists of Michigan, and shows conclusively that 
were once igneous rocks, often volcanic flows, sometimes tufas, in 
- which foliation and mineral changes have been produced by dynamic 
agencies. The rocks from which the schists were formed were diabases, a | : ; 
diabase-porphyries, diorites, and gabbros among the basic types, 
granites and quartz-porphyries among the acid types. The resulting 
3 Bull. No. 62, U. S. Geol, Survey. 
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