92 



GEOLOGY OF THE THIRD DISTRICT. 



These latter are somewhat numerous, thin, concretionary, and alternating in the bank with 

 slate for ten to fourteen feet. Back of the bank, and rising about five feet above it, are 

 thicker layers, which are quarried. When the layers are thick, the limestone is more pure ; 

 some of it is oolitic, and contains cylherina;, and the surface of the upper layer is thickly 

 covered with Orthis bicostata. Some of the limestone is of a yellowish color, with cavities, 

 and greatly resembles some of the varieties found at Niagara falls. 



From Skanandea to Verona, the concretionary mass appears in the road in all its varieties 

 as to fonn and size, exliibiting concretions whose parts are more or less concentric ; others in 

 parallel and diverging parts, regular and confused; and others in large circles like rings, 

 from the removal of the central portions, o^ving to the upper parts having been worn away. 

 It is very ochery, more so than elsewhere ; the yellow stains due to the alteration of pyrites, 

 which often discolors the rock in its other localities. 



14 



This locality is of interest also from the light which the concretions, either when whole or 

 more or less broken up, tlurow upon the cause of those appearances which disturbed or 

 deranged rocks so frequently present, the greatest diflference being in the scale merely of the 

 two. It is evident that when expansion takes place in a mass, the greatest resistance being 

 lateral as in this rock, it will rise or be forced upwards in parts, with a convex or dome-shaped 

 form, a section of which will be like one of the curves of wood-cut No. 1 . Curves of the 

 most graceful kind are exhibited in Pennsylvania and Virginia, and upon a magnificent scale 

 in the sandstone portion of the Hudson river group, evidently due to an enormous lateral 

 pressure which caused the central part to rise upwards as in the concretionary rock. The 

 same kind of curvature was seen in a limestone mass upon a very large scale in the Alps, on 

 a back road from Bex to Sion. 



In other instances of the concretionary rock, the action was more violent or the resistance 

 less ; parts were broken up into fragments, and thrown up in the form of an imperfect arch 

 as in No. 2. And again others are more disturbed, as in 3 and 4 of the succeeding wood-cut, 

 and are not unlike the appearances of ice when a frozen river or stream is broken up by a 

 freshet, the ice being packed up in all conceivable modes of arrangement. 



