i8o The American Geologist. ^i^rch, 1901. 
series. Probably, also, sedimentation was prolonged through the 
Ordovician, and the detritus has since been removed. 
Not later than early Devonian time the strata had been sharply 
folded, and were intruded by an igneous series almost entirely acid. 
The theory advanced for its origin is melting of the base of the 
slates, due to the blanketing effect of prolonged deposition ; and 
formng an acid magma above the abyssal basic one. No estimate of 
the thickness of this stratified cloak is given ; but to produce the 
existing effect there would be required a depth of sediment far 
greater than we have any warrant for inferring, and especially, such 
view seems inconsistent with the almost surface character of the 
igneous mass, not only near its summit but far down the sides, as 
exhibited in the aporhyolytes. It is much more probable that the 
intrusion burst through the base of the Cambrian series 
In cooling, the batholith became differentiated both in texture and 
composition, being finer and more basic toward the periphery. For 
explaining the latter, the author uses Becker's theory of fractional 
crystallization. The results are two types of granites, biotitic and 
hornblendic, forming the main mass ; and at the margins three 
phases, dioryte, fine granite, and aporhyolyte, the last grading into 
the granite. The later stages of refrigeration were apparently ac- 
companied by further dike intrusion of granite, and dikes and flows 
of aporhj'olyte. 
The Carboniferous sediments, which make up a large part of the 
present basin, are thought to be really one formation, deposited 
delta-like, during slight oscillations ; and while coal was being de ■ 
posited elsewhere, the bottom here was too deep. This last con- 
clusion is not borne out by the fossils discovered recently by Messrs. 
Burr and Burke ; nor by the large amount of coarse sediment present, 
much of which can be proved to have traveled but a short distance. 
Very likely the reason is to be sought rather in the almost open-coast 
conditions, by which the sea action was too rigorous. The melaphyr, 
which is so characteristic of the southern part of the basin, is con- 
sidered as a series of contemporaneous flows, and never intrusive. 
It may be pardoned, perhaps, if some students of the region still re- 
gard the evidence as pointing to intrusion. 
The Appalachian revolution gave the present structure to the 
basin as a whole, which, however, then extended far bej'ond its pres- 
ent limits. The larger faults, particularly on the margin, result each 
from two disturbances — one during the deposition of the Carbonifer- 
ous sediments, the later at the close, during the general orogenic ac- 
tion. The result, according to Prof. Crosby's interpretation, is a 
grabcn of sediments between two crystalline walls. 
As far as know, the region has been above sea level since Car- 
boniferous times, except during a portion of the Pleistocene. 
Erosion has removed the strata on either side of the graben, and 
much of the comple.x and basin. The granite has proved less re- 
sistent than the aporhyolyte, and the slate than either. The Blue 
