92 
PALAEONTOLOGY OF NEW-YORK. 
This great limestone belt marks a period of cessation in the sedimentary ac¬ 
cumulations, and may afford the means of subdividing these older crystalline 
formations. At the same time, its occurrence everywhere shows that no subsequent 
action has operated to remove any appreciable portion of the calcareous material. 
It must always happen that in the temporary or permanent change in the 
direction of the transporting current, there will be calcareous accumulations of 
greater or less importance, which will ultimately become involved in the meta¬ 
morphism of the mass. At the same time, if from any cause there should be a 
cessation of these deposits for any considerable period, then calcareous matter 
would be accumulated over that portion of the ocean bed before swept by the 
transporting current. Notwithstanding, however, all these exceptions, the pro¬ 
portion of calcareous matter in highly metamorphic regions must always be 
comparatively small. 
Although when we consider the entire mass of rocks composing the Appalachian 
chain, the proportion of calcareous matter is small, there are nevertheless exten¬ 
sive and very important strata of limestone, furnishing everywhere the white and 
variegated marbles so well known and so extensively used for architectural and 
ornamental purposes. These strata appear to consist mainly of the Lower Silurian 
limestones, which, in the Chazy, Birdseye, Black-river and Trenton limestones, 
spread out far to the westward, and, though greatly attenuated, can be traced 
beyond the Mississippi river. Their united thickness in the east, though amounting 
to several hundred feet, is not altogether one-tenth of the thickness of the sedi¬ 
mentary deposits which immediately follow, and which, in variety of form and 
degrees of intermixture, produce the argillaceous and siliceous strata of the 
lower portions of the Appalachian chain. The limestone of the Niagara period, 
and a devonian limestone apparently of the age of the Upper Helderberg lime¬ 
stone, are recognized among the metamorphic strata of this mountain range; but 
these are inconspicuous and essentially insignificant when compared with the 
whole, constituting less than one-twentieth, and perhaps more nearly one-fortieth 
of the great metamorphic sedimentary mass with which they are associated*. 
* I have not thought it necessary, in this notice, to enter into any discussion to prove the age of 
the sedimentary formations constituting the Appalachian chain. This I conceive has been established 
many years since, in Pennsylvania and Virginia, by the labors of the Professors Rogers and their 
assistants in the State Geological Surveys; in New-York and the adjacent parts of New-England, by 
the writer, and in Vermont by Professor Adams; while the investigations of Sir TV. E. Logan, in 
the Canada Geological Survey, have given such admirable results as to leave nothing more to be de¬ 
sired. Later investigations, now in progress in Canada, Vermont and Massachusetts, not only cor- 
