NIAGARA GROUP. 
117 
lower limestones of the Helderberg division, which in New-York are separated by the Onon¬ 
daga salt group one thousand feet in thickness, yet we can account for this apparent difference 
upon the supposition that the latter formation does not exist in England, and that the higher 
limestones come down upon the lower, or equivalent of Niagara, and the whole are recog¬ 
nized as one formation, as the same are in Ohio and the southwest. 
The fossils common to the two formations can be seen by the references in the descriptions ; 
they are: Strophomena transversalis, S. depressa, Orthis canalis or O. elegantula, 0. hy- 
brida, Delthyris radiatus, D. sinuatus, Atrypa affinis, A. imbricata, A. cuneata, Conularia 
quadrisulcata, Bumastis Barriensis, Iiomalonotus delpliinocephalus, Ilypanthocrinites de¬ 
cor us, Cyathocrinites pyriformis. 
These are the principal forms though there are several others which are very similar if not 
identical to those figured in plates 12 and 13 of the Silurian Researches. The occurrence of 
so great a number as here given, and which with two or three exceptions are all confined to 
this group, leaves no doubt of the perfect identity of these formations whatever may be inferred 
of others. 
12. ONONDAGA SALT GROUP. 
Calciferous slate, or Second greywacke with Shell limerock, of Eaton ( Canal Rocks, p. 124). 
Gypseous marls and slates (Annual Reports). 
[ M. of Woodcut, page 27.] 
Succeeding the Niagara group, we find an immense development of argillaceous shales and 
marls, with shaly limestones, the whole embracing veins and beds of gypsum. The general 
aspect is a light ashen, or approaching to drab color, with some portions of dull bluish green. 
The lower part is of deep red with spots of green, very closely resembling some of the more 
shaly portions of the Medina sandstone (as before described at Lewiston and other places). 
Succeeding this, where penetrated beyond the reach of atmospheric influences, the rock is 
greyish blue like the ordinary blue clays, with bands of red or brown. This portion and that 
succeeding it are .often greenish and spotted, and contain seams of fibrous gypsum, and small 
masses of transparent or reddish selenite and compact gypsum. From this it becomes gra¬ 
dually more greyish or ash-colored, with thin strata of argillaceous limestone, which is some¬ 
times dark, though generally of the same color as the mass around. The whole terminates 
upward by a grey or drab limestone, called by Mr. Vanuxem the Magnesian deposit. 
This group receives its name from the great development of the products in the county of 
Onondaga. In an economical point of view, this is one of the most important groups in the 
system ; containing all the workable beds of gypsum in Western New-York, and giving rise 
