130 BULLETIN OF THE 
contact with the scoriaceous upper surface of the lower trap, the rock 
is sufficiently coarse to detect porphyritic plagioclase crystals; but 
amygdules are entirely wanting. Under the microscope a few pseud- 
amygdaloidal areas are seen. The rock shows evidence of an original 
glassy base, seen in the triangular areas between ledges of feldspar ; it is 
made up of triclinic feldspar, magnetite, and occasionally a minute grain 
of olivine. There is a slight local tendency toward a porphyritic struc- 
ture ; but this is lost ten feet above the lower trap. Calcite and chlorite, 
the usual decomposition products, occur at the base, the latter being 
sufficiently abundant to give the rock a greenish color next above the 
red seam; this is lost ten feet above the contact, and the great mass 
of the upper sheet is of a very dark bluish color and _holocrystal- 
line. The mineralogical composition given above is that of a normal 
diabase, the amount of olivine being so small that it can hardly be 
classed as an olivine-diabase. While the upper surface of the lower 
trap is abnormally scoriaceous, the base of the upper sheet is abnor- 
mally coarse and free from vesicles, as compared with other trap 
sheets resting on sandstones or shales. This can be explained by the 
well known poor conductivity of volcanic scoriaceous substances, whose 
presence here permitted the upper trap to cool and solidify slowly, 
and produce a more complete crystallization. A practical illustration 
of the low conductivity of such material is found in the use of scum or 
slag from iron furnaces as a packing for steam pipes. 
The lower sheet may be confidently called an extrusion, but as far 
as this quarry goes, there is nothing to determine the origin of the up- 
per sheet. This, however, is fully settled by the general field evidence 
of the region, which correlates this whole mass with the heavy sheet of 
Lamentation Mountain, and that sheet has been clearly shown to be 
extrusive. 
The field evidence here referred to concerns the occurrence of faults, 
which, as is so generally the case, are at nearly all points buried under 
surface waste. It is therefore of particular interest to examine the 
bands of breccia (¢, ¢, c, Fig. 18) by which the quarry is characterized, as 
they are best interpreted as small examples of the great dislocations by 
which the structure and topography of the formation are deciphered. 
The breccias therefore deserve attentive study. The apparently unbed- 
ded sandstone, of which they in good part consist, is best interpreted as 
a fine clastic filling of the fault fractures, derived from above, where the 
walls were of sandstone or shale, and gradually filtered down among the 
large and small angular blocks of trap that were broken from the quarry 
