1884.] The Crystalline Rocks of the Northwest. . 999 
several times by different names, but on different hypotheses of 
structure, origin, and parallelism. Respecting the horizon known 
as “ Laurentian” there is an approach to unanimity and agree- 
ment. This, however, consists more in a tacit consent to style 
the lowest known rocks Laurentian, than in any agreement among 
geologists as to the nature and composition of the strata. The 
Taconic of Emmons, which has been buffeted and combatted from 
marble group), so as to include all the crystalline rocks lying 
above that group, spreading from the Laurentian to the unchanged 
Sediments of the Upper Cambrian. This has in some cases be- 
fome so obviously wrong, and has included groups of rocks so 
plainly extra~Huronian, that a double and triple nomenclature has 
en applied to.a part of these upper rocks, for the purpose of 
relieving the term of the heterogeneous burden which it was 
otherwise compelled to carry. These new names, with the ex- 
ception of the name Montalban, seem to be of value only as re- 
gional designations, the strata which they represent being igneous 
or metamorphic, and hence liable to be wanting in some places 
and to be non-crystalline in others. They further complicate the 
stratigraphic nomenclature, since they are probably only the lo- 
ad modified lower parts of the New York system. Their geo- 
graphic distribution in the Northwest not only indicates their 
stratigraphic horizon but also their limited and local existence. 
n conclusion, the chief points brought out in this discussion 
may be stated more concisely : 5 
_ 1- The crystalline rocks of the Northwest are comprised under 
six well-marked, comprehensive groups. 
3. The Huronian of Canada is the equivalent of the lowest of 
the Taconic groups, and the perfect parallel of only the lowest of 
. + ihe uppermost of the groups in the N l 
its existence, and exceptional in its characters, and has received 
‘relore a variety of names. sae 
- there is, therefore, confusion and conflict of authority in the 
application of names to the crystalline rocks of the Northwest. 
