REVIEWS 383 
is a great slate and schist series, not separable into individual forma- 
tions, and whose thickness cannot be approximately estimated. All of 
these formations have been cut by igneous rocks of various kinds and 
at different epochs. 
Metamorphism has greatly altered the character of the Algonkian 
rocks. In the Lower Huronian the quartzite is the altered form of a 
sandstone and: conglomerate in which the pebbles have been nearly 
destroyed. It is in places schistose. The dolomite is a nonclastic sedi- 
ment. ‘The slate or schist isan altered mudstone. The volcanic forma- 
tion is perhaps the most characteristic feature of the Crystal Falls 
district. It occupies a larger area than the other Lower Huronian 
formations and consists of basic and acid rocks, lavas and tuffs, with 
subordinate interbedded sedimentary rocks. The iron-bearing forma- 
tion, cailed the Groveland, consists of sideritic rocks, cherts, jaspillites, 
iron ores, and other varieties characteristic of the iron-bearing forma- 
tions of the Lake Superior region. 
After elevation and unequal erosion of the Lower Huronian, con- 
ditions of deposition covered these formations with sandstone and 
slate conglomerate, passing upwards into shales and grits, subse- 
quently altered to mica-slates and mica-schists. These were followed 
by combined clastic and non-clastic sediments, the latter including 
iron-bearing carbonates. Above these is a great thickness of mica- 
slates and mica-schists. 
After a long period of deposition a profound physical revolution 
occurred, raising the region and folding it in a most complex manner. 
The folds have steep pitches indicating great compressive stresses in 
all directions tangential to the surface of the earth. Subsequent to or 
during the late stage of this time of folding there was a period of great 
igneous activity, probably contemporaneous with the Keweenawan, 
intruding within the rocks vast bosses and numerous dikes of peridotites, 
gabbros, dolerites and granites. These intrusives, while altered by 
metasomatic changes, do not show marked evidence of dynamic meta- 
morphism. 
Subsequently the region was subjected to great denudation and 
reduced approximately to its present configuration. In late Cambrian 
time Upper Cambrian sediments were deposited upon it. Whatever 
may have been deposited upon the Cambrian has been removed by 
erosion together with most of the Cambrian. If the region was again 
submerged in Cretaceous times no evidence of the fact remains. , 
