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 nest 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 irom iron furnaces as a packing foi' steam pipes. 



The lower sheet may be confidently called an extrusion, but as far 

 as this quarry goes, there is notlihig 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, c, Fig. 18) by which the quarry is characterized, as 

 they are best interpreted as small examples of the great dislocations b}' 

 which the structure and topography of the formation are deciphered. 

 The hreccias 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 



