334 Review of Recent Geological Literature. 
sometime before its publication (1847 ? ), was not yet satisfied 
of the existence of two separate formations of slates in this 
area. He speaks of the " Hudson slates " as lying nearly hor- 
izontal sometimes, above the Trenton limestone, and being 
but a thin and unimportant formation ; at other places he 
says they stand nearly on edge and are several thousand feet in 
thickness, instancing their extent through Rensselaer and 
Washington counties, in Vermont, and through the entire 
length of Lower Canada. The " Black Slates," at the falls of 
Montmorenci, he plainly intimates he considers as belonging 
above the Trenton, although his figure represents them as 
lying below it. He supposes their lower position here to be 
due to a " down-heave," by which the slates were thrown at 
the same time into an inclined position, and placed uncon- 
formably on the underlying "gneiss," their absence above 
the Trenton at the same place being due to erosion in 
the lapse of time. In this interpretation he is supported by 
the later observations of Mr. Selwyn, but not by those of Mr. 
Marcou who maintains that the slates that lie adjacent to the 
gneiss (?) in an inclined position really are no part of the 
Trenton, but belong to the primordial zone. Mr. Lapworth, 
who has examined some graptolites from these inclined slates, 
seems inclined to the same view. 
There is nothing unreasonable in the supposition that the 
Trenton overlies unconformably some tilted primordial beds 
at this place, and if the evidence of other observations on the 
position of the Trenton be appealed to even within the St. 
Lawrence valley, it is necessary to admit that the Trenton 
epoch was one of such subsidence that the rocks of that age 
were laid down unconformably on strata still older than the 
primordial. Such a position could not be attained by these 
beds had they not been brought, by the same overlap, uncon- 
formably also over the primordial. 
REVIEW OF RECENT GEOLOGICAL LITERATURE. • 
Examination of water for Sanitary and Technical Purposes. By Dr. 
H. Lepfmann and Wm. Beam. 106 pp. 12nio. 1889. Philadelphia. 
(Blakiston, Son & Co.) 
The editors give clear and concise statements of the most approved 
methods for the analysis of water for the purpose of determining its 
fitness for domestic, boiler and other industrial purposes. 
