706 C. K. LEITH 
the Baraboo quartzite in Wisconsin, which has been referred by Irving 
and Van Hisetothe Huronian. The diabase intruding the slate, supposed 
to be the upward continuation of the quartzite, is strikingly similar to 
intrusives which are peculiar to the Huronian in the Lake Superior region. 
Frazer* sketches the geology of an area in the vicinity of Galena, 
in the Northern Black Hills of South Dakota. Mica-schists, thought 
to be upper members of the Archean, are found striking northeast- 
southwest, and dipping at angles from 38° to 85°. ‘They are gen- 
erally micaceous and coarse-grained, but vary greatly, sometimes 
passing into nacrite- or hydromica-schist, and sometimes, though more 
rarely, assuming a heavily bedded character reminding one of gneiss. 
Todd? reports on a section across the Black Hills from Rapid City 
westward. The alternating slate and quartzite beds of the Algonkian 
were found to be folded in a most intricate fashion. A number of the 
folds were worked out. In most cases the lamination and stratification 
seem to correspond in direction. 
Comment.—Vhe last observation differs from one made by Van 
Hise, who, as a result of work done in 1890, concluded that the 
prominent foliation of the Black Hills is independent ot the bedding, 
and as a rule cuts across it. 
Griswold? describes the geology of Helena, Montana, and vicinity, 
Middle Cambrian, or Flathead, quartzite forms an important part 
of the ridge stretching from Helena southeast to Montana City, and 
northwest, west, and south around Mount Helena. The sedimentary 
rocks underlying most of the area of the city, on the north side of 
this Cambrian quartzite, are classed as Algonkian. The Algonkian 
rocks vary from clay-slates to micaceous, sandy, or calcareous slates, 
which often become quartzites or limestones. The Algonkian slates 
seem to conform to the overlying strata in the dip of their beds. As 
there are many small folds, it is difficult to determine the thickness ; 
5000 feet does not seem too large a total. 
Gilbert* maps and describes the geology of the Pueblo quadrangle, 
* Notes on the Northern Black Hills of South Dakota, by PERSIFOR FRAZER: Trans. 
Am. Inst. Min. Engineers, Vol. XX VII, 1898, pp. 204-228. 
? Section along Rapid Greek from Rapid City westward, by J. E. Topp: South 
Dakota Geol. Survey, Bull. No. 2, 1898, pp. 27-40. 
3 The geology of Helena, Montana, and vicinity, by L. S. GRIswoLD: Journal of 
the Association of Engineering Societies, Vol. XX, 1898, pp. 1-18. 
‘Geol. Atlas of the U. 5., Pueblo folio, No. 36, by G. K. GILBERT: U. S. Geol. 
Survey, Washington, 1897. 
