228 DR. THOMAS STERRY HUNT ON THE 
bedded deposits of zinc-blende, and oxydized ores of zinc are found at the outcrops of these. 
The chief facts in the mineralogical history of these rocks will be found in my volume 
already cited ; Azoic Rocks, etc., pp. 201, 206. See also a description of the Cornwall Iron- 
Mine, etc., Proc. Amer. Institute Mining Engineers, vol. IV. pp. 319-325, and two notes 
on The Taconic System, and on the Genesis of Iron Ores, published in the Canadian 
Naturalist for December, 1880; besides a farther discussion of the subject in an essay on 
The Decay of Rocks Geolog awe Considered, read before the National Academy of Sciences 
at Washington, April, 1883. 
§ 27. As regards the eee of the strata which in the central region of Pennsylva- 
nia underlie the sandstone No. IV:, we find in the Kishacoquillas and Nittany valleys, 
respectively ; for the Loraine shales a thickness of 1,200 and 700 feet, for the Utica slate, 400 
and 300 feet, and for the fossiliferous Trenton beds, 400 and 300 feet. Beneath the latter, in 
these central valleys, lies a great mass of magnesian limestones interstratified with schis- 
tose beds; the whole called by Rogers, Auroral, and supposed by him to be, like the similar 
rocks in the eastern part of the state, the representatives of the Chazy and Calciferous 
divisions of the New York system. To this succession of limestones, as observed at Belle- 
fonte, Rogers assigned a thickness of over 5,400 feet, of which the upper 600 are highly fossili- 
ferous ; while the great underlying portion is destitute of fossils, or contains but few and 
undetermined organic forms. The most complete section of the strata below the sandstone 
No. IV.,in the central region, is that lately measured by Mr. Saunders, in Blair county; where, 
beneath 900 feet representing the Loraine and Utica shales, are found not less than 6,600 
feet of strata, including, at the top, the fossiliferous Trenton beds, whose thickness is not 
separately given, and, near the base, intercalated sandstones and shales. A summary of 
this section gives, in descending order :— 
Feet 
Sandstone No. IV......... Feu de RAS tn dE adacpabsscaccspcqnn co Resa le ere 
Wipperyshallesi(WiticayandeWorsine) ere ee ee 900 
Limestones and dolomites, including the fossiliferous Trenton...... ........ 5,400 
AI AMIE SAN ASTON eee ne eee eee ee eee eee 40 
Limestoneswith sandstone and! ShAleS.... seems ee semmc eee eemeessecee 1,160 
7,500 
$ 28. In none of the sections of these rocks exposed in the eroded anticlinal valleys 
of the west, has anything been found corresponding to the older crystalline groups which, 
along the border of the south-eastern region, underlie the base of this series. For the rest, 
these lower non-fossiliferous strata present similar mineralogical characters to those of the 
great valley to the south-east, and include extensive deposits of limonite (imbedded in 
clays, which are decayed schists in situ), as well as ores of zinc; both of which are largely » 
mined in Blair county. For the above details of measurements see report T. of the 
second geological survey of Pennsylvania, by Franklin Platt, pp. 18, 48-59. 
§ 29. If we turn from these central valleys of Pennsylvania to what Rogers called the 
south-eastern area, that is to say the regions lying to the south-east of the Kittatinny 
Mountain, we find a very different condition of things. In place of the 900 feet of fossili- 

* American Journal of Science (3), xxvi., 201-205. 

