326 
GEOLOGY OF THE FOURTH DISTRICT. 
These markings range from the slightest possible scratch to grooves of half an inch in 
width, and from one-eighth to one-fourth of an inch in depth. Such grooves could only have 
been made by some hard substance, moved with great force, and under great pressure, over 
the surface of the strata; for not only do we see the deep grooves and shallow striae, but 
fragments are broken out as we approach a fissure in the stratum, as if crushed by some heavy 
body. Such phenomena, which are frequently witnessed, are illustrated in the following 
woodcut, where the lower side of the specimen, as shown in the cut, is broken off obliquely, 
so that the upper surface recedes six inches beyond the lower. This took place upon the 
southern edge of a mass at the crossing of a fissure. The mode of fracture indicates an im¬ 
mense weight, pressed upon the surface, but not a quick, heavy blow. The grooves follow, 
somewhat obliquely, the fractured slope, which was probably made near the close of the ope 
ration; for, in many instances, such surfaces are afterwards polished, but this one retains its 
freshness. There are in this specimen (and the same often occurs in others) two sets of 
striae; one, being the prevailing direction, is as stated above, while the other is very nearly 
north and south. 
161 . 
The outcropping edges of strata, previously polished and grooved, are often overturned 
upon the rock, in place, by this force moving southward. Upon the surface from which the 
above specimen was selected, I have seen frequent examples of this kind; and in some in¬ 
stances the fragment is partially rounded, possessing the character of a boulder. At this 
locality the grooves are often large and deep, while in other places they are fine, and the sur¬ 
face is nearly polished. The materials resting above this rock are fine sand and sometimes 
clay ; but in no place in the same neigborhood have I seen gravel, and only occasionally large 
fragments of the rock itself from the outcropping edges. A few miles farther south, however, 
there is a great accumulation of gravel, covering the strata. 
