162 The American Geologist. September, 1894 
Archaean age. The basis of this assumption as to the age of 
these rocks is evidently due to their association with the 
gneisses and highly metamorphosed rock which form so prom- 
inent a feature of this region, together with their highly 
crystalline condition, and to the general absence of fossils. 
Upon such evidence, their Archaean age has not only been 
generally accepted, but considered so positive a fact by many 
authors that such features as the}- present have been used as 
a factor in determining the supposed Archaean age of lime- 
stones elsewhere, and, in general, as proof positive of pre- 
Bedimentary limestones. 
This determination of these and other crystalline areas was 
quite natural and consistent at the time it was first made in 
New York and New Jersey; for then the whole sedimentary 
series was terminated below by the Potsdam sandstone, and 
everything older was considered as belonging to the Azoic. 
With the progress of geological science, and after the estab- 
lishment of the Cambrian or Taconic system of rocks, one by 
one these pre-Potsdam sedimentaries have been rescued from 
the Azoic or Archaean and their Cambrian age established by 
means of fossils. In general there remain undetermined at 
the present time only the more crystalline and metamorphic 
regions generally not directly connected with unaltered sedi- 
ments of known age. In many cases the crystalline areas 
shade off into fossiliferous strata in undisturbed regions and 
their correlations have been determined. 
It thus appears that many areas of altered sedimentary 
rocks, such as crystalline limestones, schists and quartzites 
still remain in the Archaean, simply because they cannot yet 
be correlated with any know r n horizon. This is in itself no 
evidence of their true relationships, and is inconsistent with 
proper methods of geological correlation. It is a curious fact 
that the assumption having once been made that these rocks 
were of Archaean age, the lithological features that they dis- 
played, especially in the presence of chondrodite, gradually 
came to be regarded as proof in itself of their Archaean age. 
These crystalline limestones have, since 1864, been de- 
scribed in the various reports of the Geological survey of New 
Jersey as members of the series of crystalline rocks of Azoic 
or Archaean age. In INKS and 1889 the writer was engaged 
