Review of Recent Geological Literature. 347 
jointed that the industry has not yet proved to be profitable. Large 
areas, fifty miles in extent, are supposed to be underlain by such slates. 
This is a grand feature which allies the Holyoke slates with the Thom- 
son (Taconic) slates in Minnesota, and with the supposed "Lower Cam- 
brian"' slates of the Huronian region in Canada southeastward from 
Vermilion lake in the vicinity of Sudbury. 
The hypothetical granite and felsyte of the Holyoke formation con- 
sist now of schistose rock, mainly chloritic, cutting the bedding in the 
form of dikes, and they may instead have originated from "porphyrytes 
or diorytes." 
Peridotyte, serpentine and dolomyte, placed by the report in the 
Holyoke, compose that rock seen at Presqu' Isle and northwestward 
from Ishpeming, and these rocks, constituting a geological unit, are re- 
garded as the "youngest of the large intrusive masses seen thus far in 
the Marquette district." The writer,on the other hand, considers them 
a constituent part of Dr. Wadsworth's Republic formation,but not altered 
eruptives in the usual acceptation of that term. The materials of which 
they are composed were water-stratified, but of the nature of volcanic 
tuff, originating in Archsean time and "altered" in Archaean time 
by the waters of an Archaean ocean. 
The melaphyr and picryte are problematical, schistose rocks of a dirty 
brownish or grayish color seen on the islands north of Marquette, sup- 
posed to be "altered" andesytes or peridotytes. The dikes of diabase 
and melaphyr are the youngest of the eruptive rocks of the region, and 
are not known to cut the "Eastern sandstone." One such dike is known 
(on Presqu' Isle) to be overlain by undisturbed, horizontal "Eastern 
sandstone." 
According to observations made by Mr. A. E. Seaman in the vicinity 
of Bessemer, and in T. 46-41, there seem to be two sandstones closely 
associated with the diabases of the Keweenawan, one of which is indur- 
ated, conglomeratic, rests on steeply inclined green schists, and in 
places is a quartzyte; the other is near adjacent, friable, with spots of 
deoxidation like the "Eastern sandstone" and contains pebbles of mela- 
phyr and large, angular fragments of indurated sandstone which he con- 
siders derivable from the indurated sandstone outcrop adjacent. If 
this indurated rock be not the underlying quartzyte of the iron-bearing 
series of the Taconic, Mr. Seaman's reasoning tends to show the occur- 
rence of a break and an erosion interval in the progress of the Keween- 
awan epoch, of a character quite different from the intervals which 
separated the eruptive episodes which characteristically composed the 
history of the lower portion of the Keweenawan. 
There is a chapter devoted to the "chemical deposits of the Azoic sys- 
tem." The soft iron ores and manganese deposits are thought to be 
due to chemical changes in the original rocks brought about by » the ac- 
tion of percolating water. The author considers that "rocks of entirely 
different origin and structure, like sedimentary and eruptive rocks, 
have been so changed by this action that the resulting forms are indis- 
tinguishable from one another except by their geological mode of 
