58 
[Assembly 
Cambrian or Hudson system, rest in a nearly horizontal position the 
Silurian strata, with the limestones and slates of which series commenc- 
ed the first condition of the seas, favorable to the existence of myriads 
of shells, corals, and trilobites, whose exuviae have very materially add- 
ed to the thickness of the strata. 
In the report of the geologist of Pennsylvania, the olive sandstone 
of the Cambrian or Hudson strata, has been confounded with the fourth 
rock of the Silurian system, known by the name of Salmon river sand- 
stone, which formation is admirably characterized in New- York, Penn- 
sylvania and Ohio, by the Pterinea carinata of Goldfuss. 
There is strange misunderstanding of the method of applying orga- 
nic remains in the division of series of strata into formations, and the 
identification of widely separated rocks by the Zoological characters of 
each. In the January number of the New- York Review, the opinion 
is advanced, that the " Calymene Blumenbachii ought to be carefully 
sought for in the rocks which are said to correspond to the Dudley pe- 
riod. Unless it is found, or some other consideration is introduced, can 
it be believed that fossils are a satisfactory evidence of the age and place 
of rocks^" The line of demarkation between rocks of different age 
has never yet been drawn with any accuracy by the aid of Palaeonto- 
logy, except by the consideration of groups of species, one, or even a 
few species, having no weight whatever in the determination. Thus the 
shell termed Terebratula Schlotheimii, dates its existence with the Tren- 
ton limestone, and reappears in three of the latter formations of the Si- 
lurian system. Orthis testudenaria, Dalm. is peculiarly characteristic of 
the Trenton limestone, by its almost invariable presence and extreme 
abundance, and yet it is also found in the limestone of the Helderberg 
a formation of a far more recent origin. But, although a few species 
may have been continued through a succession of geological eras, the 
groups are widely distinct, and their value in determining the compara- 
tive age and identity of formations, can never again be called in question 
by a geologist who deserves the name. 
Excepting two trilobites, I have always found the organic remains of the 
Dudley limestone to correspond with those of rocks far above the Tren- 
ton limestone, and therefore it was natural to suppose that the Calymene 
Blumenbachii had escaped from the catastrophe in remote seas, which 
destroyed it here. But that trilobite has lately been found in the Ro- 
chester shale, in company with Asaphus caudatas; in this place it oc- 
cupies the same position in the scale of formations which it holds at 
