30 
GEOLOGY OF THE FOURTH DISTRICT. 
6. Hudson-river Group. 
Where the strata are undisturbed, a well marked line of division usually separates this group 
from the Utica slate; but along the Hudson river, and in other places where disturbance has 
prevailed, the two are not easily separable. Indeed from the fact that several fossils of the 
Trenton rocks are continued through the Utica slate, and appear in this group, we might al¬ 
most be inclined to consider it as a continuation of the same ; beginning with a shaly limestone, 
and passing through shale and shaly sandstone to the termination of the series. The group 
consists of shales and shaly sandstones, with thin courses of limestone, and in many places 
its upper portions abound in fossils. 
Organic Remains. — The following are those enumerated and described : f Calymene 
senaria ? Triarthrus Beckii, Trinucleus caractaci, Avicula demissa, Pterinea carinata, Cypri- 
cardia modiolaris, C. angustifrons, C. ovata, Nucula, Strophomena nasuta, t S. sericea, 
Strophomena-? f Orthis striatula, O. Actoniae, O. testudinaria ? O. crispata, Orthoceras 
cequalis, Pleurotomaria, stems of Crinoidea, Graptolites serratus, G. scalaris, &c. It will 
be seen that several species ( f ) are common to this group and the Trenton limestone. 
This group appears to be equally extensive with any of the lower masses, but its lithological 
characters change essentially at distant localities. Upon the Hudson river, and in most other 
places in New-York, it is exhibited in the form of slates and thin-bedded shaly sandstones, 
forming what has been known as argillite and greywacke. Fursuing this group westward, it 
is found in Ohio represented by limestone and shale or marl, forming the “ Blue limestone” of 
the Geological Reports of that State. It presents the same character in Kentucky, Indiana 
and Wisconsin. At the same time, however, it retains many of its characteristic fossils, with 
the addition of many new ones, or species which do not exist in any group in New-York. 
This group thus exhibits an example where lithological character has ceased to be of any great 
importance in identifying strata. The fossils at the same time are found to be much more 
constant, though the greater number of individuals of different species renders the eastern 
types less prominent than otherwise. 
This group not only extends into Canada on the north and east, but far west and southwest; 
appearing, if we may judge from specimens, in Tennessee, and near the Hot springs in 
Arkansas. It is likewise an important rock in Pennsylvania and Virginia. This mass be¬ 
comes interesting from the fact that it appears to be equivalent to the second great group in 
the Silurian system of England, representing in its undisturbed and fossiliferous condition 
the Caradoc sandstone. This would appear from the comparison of numerous specimens, 
brought out by Mr. Lyell from well known localities in England, and which are now in the 
Collection of the Lowell Institute at Boston. These leave no doubt of the perfect identity 
in fossil and lithological characters. The same group, when disturbed and upheaved, as it is 
on the Hudson river, doubtless represent the Cambrian system ; this portion having generally 
been considered as a much older series than the same farther west. But the facts correspond 
to those observed in England, viz. an identity in the contained fossils ; the only difference 
being produced by disturbance, and sometimes a partial alteration of the products. 
