Revieiv of Recent Geological Literature. 309 
nominee district to the southeast. It is nevertheless of much interest 
from an economic as well as from a geological standpoint, for in it 
are in general the same iron-bearing formations as in the two adjacent 
mining districts, and a study of this intervening area (the Crystal 
Falls) makes it possible to correlate the strata in the two important 
districts. The equivalence of the rocks of these three districts can 
now be established with a considerable degree of certainty, although 
the sequence in each district is not precisely the same as in the others. 
In general in each district the I^ower Huronian is represented by a 
lower quartzite overlain by a dolomite and this in turn overlain by 
an iron-bearing member which forms the upper part of the Lower 
Huronian. The sequence in the Upper Huronian has not been so 
fully worked out, but this formation is present in each of the three 
districts. 
Heretofore comparatively little concerning the geology of the 
Crystal Falls district has been known, owing to infrequency of ex- 
posures and its unimportance as a mining district when compared with 
the adjacent Marquette and Menominee districts. The present paper, 
which is an abstract of a monograph (No. XXXVI. of the Mono- 
graphs of the U. S. Geological Survey), gives in condensed form an 
excellent description of the geology of the district. On the east of 
the Crystal Falls area is a large mass of Archean rocks, largely gra- 
nitic in character and at times schistose. To the west of this large 
mass are two smaller masses of Archean, oval in shape and about ten 
miles long (north and south) by two or three wide. Resting un- 
conformably upon the Archean and with the uncomformity sometimes 
much obscured by metamorphism, are the Lower Huronian rocks 
which in ascending order are as follows: Sturgeon quartzite, Rand- 
ville dolomite, Mansfield (slate and schist) formation. Hemlock (vol- 
canic) formation, and Groveland (ferruginous) formation. Uncon- 
formable above the Lower Huronian is an unknown thickness of un- 
divided Upper Huronian rocks consisting essentially of conglomer- 
ates, quartzites, grayv/ackes, several varieties of slate, and ferruginous 
rocks. Of later date than all the foregoing are considerable masses 
of igneous rocks, which are of a basic nature — diabases, gabbros and 
diorites. These are probably of Keweenawan age. Overlying all are 
remnants of horizontal Cambrian sandstone. 
At the end of Lower Huronian time the region was subjected to 
folding and again at the end of Upper Huronian time, the latter fold- 
ing being in places very intense so that the strata are now not infre- 
quently nearly vertical. The folding and the intrusion of igneous 
rocks, which took place later than the last folding, induced meta- 
morphism which varies much throughout the district. Two interest- 
ing features of this metamorphism may be mentioned. One is the 
production of a conglomerate-gneiss, the matrix of which has been 
completely recrystallized, but which still contains numerous flattened 
pebbles and boulders. The other is the production, from some of 
the Hemlock volcanic rocks, of completely crystalline schists which 
