1864.] 4G9 [Lesley. 



the Tertiary age of the iron ore beds containing lignite. In fine, it 

 is the main fact of the discussion. 



The gradual lowering of the main surface-plane involved, 1st, the 

 obliteration of all grand original inequalities which would have 

 been produced by a grand original cataclysm, if a cataclysm be al- 

 lowed ; and, 2d, the production of a new set of inequalities, due 

 partly to structural relations of movement, such as folds and faults, 

 but chiefly to the different liomogeneousness, the different compactness^ 

 and the diffei'ent msolitbilif^ of the formations. These three chemi- 

 cal and lithological differences, of course, produced our present moun- 

 tain, hill, and valley surface. It is evident, then, that the reason why 

 the southeast side of the Gi'eat Valley is everywhere lower than the 

 northwest side, is because it represents the more soluble and less 

 compact outcrop edges of the Lower Silurian limestones No. II, while 

 the other, or northwest side of the Great Valley, consists of Lower 

 Silurian slates No. III. In fact, the Great Valley may be said to be 

 as to the northwest half of it paved with low hills. These are 

 the slate hills of that half of the valley which lies up against the 

 North (Kittatinny, Blue or Brush) Mountain. The southeast half 

 is a nearly perfect plain, cultivated like a garden, and exhibiting in 

 the fields numberless low ledges of limestone rock, beside many of 

 which stand limekilns. 



There are certainly evidences of some obscure nonconforniability 

 between the limestones of II and the slates of III above them ; for, 

 while the strike of the slates is always straight up and down the Valley, 

 that of many groups of these limestone-outcrops is perversely out of 

 line, often crossing the valley at various and sometimes at right an- 

 gles. But much of this apparent nonconforniability is no doubt due 

 to crimpling, although the whole formation is much more nearly 

 horizontal than it has had credit given to it for being; and much of 

 it is a deception, produced by an extraordinarily well-developed 

 system of cleavage-planes. On the whole, the regularity of the 

 bounding mountains, and the symmetry of the Valley itself, are good 

 guarantees against any serious nonconformability. 



Before the beginning, and again, after the close of the limestone 

 Lower Silurian age, there were depositions of ferruginous mud, caus- 

 ing two slate formations, a lower. No. I, and an upper, No. III. The 

 contact of the limestone and the upper slate, along the central line of 

 the Valley, is marked by a range of iron ore. In a few instances it 

 is abundant and largely excavated for the furnaces of Pennsylvania. 



The contact-line of slates just under the limestone No. II, with 



