322 TJie American Geologist. November, 1900 
REVIEW OF RECENT GEOLOGICAL 
LITERATURE. 
A Cotttribiition to the Geology of the jYorther?! Black Hills. Thesis 
Submitted in Partial FuUfillment of the Requirements for the Degree 
of Doctor of Philosophy, in the Faculty of Pure Science, Columbia 
University, June iSgg, . By John Duer Irving. [Annals N. Y. Acad. 
Sc. Vol. XII, No. 9, pp. 187-340, pis. V-XVI, figures 5-20.] 
This paper constitutes a valuable addition to the geology of the 
Black hills. Its purpose is the extension of stratigraphic and pe- 
trographic detail rather than the development of new theoretical 
knowledge. The district described exhibits most strikingly many 
phenomena whose origin is due to the vast intrusive masses which 
form the predominating rock of the region. 
The Black hills topography strikingly illustrates the relation of 
degradation to geological structure. In the western area the country 
rock consists of horizontal Carboniferous strata; in the plateau which 
these strata form streams have cut precipitous canyons, leaving fiat 
table-like divides. Farther east the topography consists of the 
conical liills and sharp irregular ridges with complicated drainage, 
characteristic of regions which have been the seat of igneous activity. 
Dr. Irving finds Algonkian, Cambrian and Carboniferous beds, 
containing many intrusions. The character of the intrusion is found 
to be dependent upon the nature of the rock into which it is in- 
truded; — in the vertical slates and schists of the Algonkian only dikes 
are found; in the soft thinly bedded Cambrian shales, are sheets; the 
massive Carboniferous sandstones and limestones are penetrated by 
very few dikes and by some irregular masses which Dr. Irving re- 
gards as belonging to none of the ordinary classes of intrusion. In 
each case the intrusion has followed the direction of least resistance 
and subsequent erosion has left the dikes standing out as ridges and 
the sheets as laccoliths. 
Petrographically the intrusives are found to present a large 
variety of types. The most abundant are the various kinds of 
I)honolyte which constitute the greater number of the laccoliths. 
Various types of rhyolytes, andesytes, dacytes, diorytes, lampro- 
phyric dike rocks and amphibolytes are found. The Black hills have 
at least twice been the seat of prolonged igneous activity. The first 
period was previous to the metamorphism of the Algonkian and 
was characterized by the intrusion of sheets of basic rock, now rep- 
resented by the amphibolytes. These intrusives were metamorphosed 
with the Algonkian strata, and together they were raised to a vertical 
position, hence the sheets now appear to be dikes. The second period 
of activity was post-Cretaceous and was marked by a highly alkaline 
series of intrusions. This series forms true dikes in the Algonkian 
and sheets in the Cambrian. It yet remains to be determined whether 
the lamprophyric dike rock (augite-vogesite) represents the final 
