White limestones of Sussex Co., JV. J. — Nason. 251 
uncertaint}* in man}' minds in relation to the age of these rocks, 
it being thought by many that they were of the same age as the 
blue or magnesian limestones, only changed by metamorphic 
action, and that the passage from one to the other was a very 
gradual one. Prof. Rogers in his Final Report on the geology of 
New Jersey, pp. 61-80, under the head of "Igneous rocks con- 
nected with Formation II," discusses the changes induced upon 
the Blue limestones by igneous action and assumes that all the 
white or crystalline limestones are made from the Blue limestones 
by the action of heat which has been applied by the agency of 
dj-kes of granite. 
In regard to the crystalline limestones he was mistaken. They 
are everywhere conformable to the gneiss and interstratified with 
it. This mistake is acknowledged by his former assistant, J. P. 
Lesley, in the American Journal of Science, vol. 89, p. 221. The 
true position and identity in age of the ciystalline limestone and the 
gneiss, was clearly proved by Vanuxem and Keating in the ' ' Jour- 
nal of the Academy of Natural Sciences " in 1822, and this view 
has been sustained by all of the observations of Dr. Kitchell and 
his assistants, and can easily be verified by airy one who will visit 
the localities cited in this report." 
This defines the position of Dr. Cook on this question. His 
opinion is evidently not founded so much upon personal investi- 
gation and observation as upon the reports of others. His proof 
hangs upon the extremely attenuated thread of conform ability, 
which is even in this case more apparent than real, and upon the 
confusion attendant upon the use of the term "gneiss." This 
opinion Dr. Cook held up to the day of his death. The errors of 
Vanuxem and Keating I have already pointed out. With regard 
to Prof. Lesley's acknowledgment of the error of Prof. Rogers 
this much only can be said : He simpl}- announces his belief casu- 
ally in reply to a criticism on Prof. Rogers' work by Hall and 
Logan, that Prof. Rogers was in error. His belief appears to 
rest upon a misunderstood statement, published in the American 
Journal of Science, 18G1, vol. 82, p. 208, by Dr. Cook. As 
Prof. Lesley quotes Dr. Cook " he (Dr. Cook) has seen horizontal 
Potsdam sandstone or Calciferous beds overlying these upturned 
Franklin limestones." 
In the first place, these beds are not horizontal, but dip steeply 
to the N. W. ; In the second place, they rest upon the gneiss of 
Franklin furnace and are nowhere near the white limestone. 
