1884.] The Crystalline Rocks of the Northwest. 99I 
a position as to overlie it, exposing a thickness of at least twenty- 
two feet. 
Now, the difficulties of the situation arise when we cast about 
to find names for these parts. What are the eastern representa- 
tives of these western groups, and by what designations shall 
they be known ? i 
Since the geological survey of New York, and the publication 
of its final report, the progress of geological science in Europe 
and America has rendered it necessary to revise some of the dog- 
mas which were regarded as fundamental by the New York geol- 
ogists, and to reject entirely some others. Among these may be 
mentioned the then current theory that the term “primary ” 
should be applied to any massively crystalline rock, and that all 
such rocks belong to the bottom of the chronological scale of 
geology. If the apparent structural relations of the forma- 
tions, as seen in the field, did not agree with this theory, some 
violent movement in the earth’s crust was at once conjectured so 
as to bring nature into accordance with the true theory. Latterly, 
however, it has been shown abundantly by Dana and others, that 
the Trenton, Hudson River and other Silurian rocks are converted 
into crystalline schists ; by Whitney that the Tertiary rocks be- 
come crystalline; by Brooks and Frazer that the Potsdam sand- 
stone becomes gneissic ; by Reusch that the clay slates, interbedded 
with the granites and gneisses of the Bergen peninsula of Nor- 
way, contain characteristic upper Silurian fossils, and by Hitch- 
Cock that the Helderberg rocks of New York are involved in the 
crystalline terranes of New Hampshire. 
These more recent crystalline series, however, can all be con- 
Sidered as excluded from the scope of search for any parallels to 
the crystalline groups of the Northwest. Our inquiry will in- 
Volve only the well-known names Laurentian, Huronian, Taconic 
Montalban, Arvonian, Norian. 
We meet at the outset with the question which has now become 
as historic in American geology as the Cambro-Silurian contro- 
versy in England, and which concerns very nearly the same geo- 
logical horizon, viz: Is there a formation such as claimed by Em- 
mons—the Taconic? On this geologists are yet divided. We 
Conceive, however, that the division is caused, not so much by 
doubt as to the existence of a sedimentary fossiliferous formation 
* Lesley, Report C4. 
