r'razer.] ^^t) [Dee. 4, 187i. 



Bound Top proper, where the Sixth Corps of the Army of the Potomac 

 intrenched during the night of the 2d and the morning of the 3d,) my 

 attention was directed to a singular example of weathei-iag which was so 

 entirely novel to me, that I determined to secure specimens of it for ex- 

 hibition to this Society as well as the Academy of Natural Sciences. 



It seems to open to me a new view of concretionary structure as well 

 as surface weathering, and is an important item for consideration when 

 the rock is intended J or bailding purposes. 



The fracture of these rocks (and indeed of all rocks) should be sub- 

 divided into — 



1. Fracture on large planes. 



2. Fractu^re on small planes. 



It is essential to know whether reference is made to large or small 

 planes when the kind of fracture is described in all rocks, for though the 

 general habit of the large plane may be a curved surface where this is 

 shown in the original boulder, the smaller fragments may exhibit splin- 

 tery, earthy, or any other fracture. 



Several of these large boulders are visible in the "Devil's Den," which 

 present 100 square yards or more of surface, and in one or two cases 

 where the fracture seems to have been recent, the surface is very homo- 

 geneous, the curve very smooth, and the rock very sound and hard, and 

 with a bluish gray color entirely different from the brown which it 

 assumes in places where it has been more exposed to the weather. 



In some of these latter specimens it would be difficult to persuade the eye 

 that the object was not a Cyclopiau wall of rounded and square blocks 

 built up by the hand of man, nor is the delusion dispelled by a close ex- 

 amination of the rock. The spaces between the apparently separated 

 blocks are seemingly in need of "pointing up," but otherwise there 

 seems to be a material at the junction different from the mass of the 

 rock. 



At one blow of the hammer a shell varying in thickness from | to f 

 inch and discolored by weathering, though not friable, falls off and the 

 surface beneath is seen to be of noimal structui-e, texture and tenacity.^ 



One curious part of this phenomenon is the tendency of the weathered 

 surface to become conchoidal, even where the face of the rock is plane. 

 It results from the gradual sinking of the outside surface towards the 

 depressions that form the divisions between the sepai'ate blocks. The 

 mode of formation of these curious false walls appear to be first, the 

 gradual solutions of parts of the Labradorite matrix between the horn- 

 blende crystals. 



Certain lines are more readily soluble than others, and these gradually 

 deepen as the troughs that are formed conduct more water over the most 

 yielding parts. The small crystals of hornblende in such troughs after 

 losing their support falls out and are washed away, and at the same time 

 the sides of these miniature troughs being constantly subjected to the sol- 

 vent action of running water and the trituration of the suspended matter 



