MUSEUM OF COMPARATIVE ZOOLOGY. 125 
particularly interesting in the way it recalls the details of the slow 
process by which these trap sheets were buried. Irving mentions the 
occurrence of filled vesicles at the upper surface of the diabases and 
diabase-porphyrites of the Keweenawan series of Lake Superior, and 
cites it as one of the strongest proofs of the extensive origin of these 
rocks.? 
Fragments of vesicular trap are not uncommon in the sandstone im- 
mediately overlying the surface of the sheet ; their edges appear some- 
what water-worn. It is of course possible that such fragments as these 
might have been derived with the sand from some distant source, and 
that they therefore do not in any way bear on the contemporaneous 
extrusion of the subjacent sheet. In such a case we should expect to 
find fragments of trap at various horizons in the Triassic series, showing 
no definite association with the intercalated trap sheets, but this is not 
the fact. The sandstones and shales throughout the valley here and 
there contain abundant fragments of trap, but, except in a few cases 
that will be specified, the fragments occur only in the bed immediately 
overlying some one of the sheets of the eastern trap ranges; the frag- 
ments are commonly vesicular, and as such cannot have survived long 
transportation ; they are moreover but imperfectly water-worn, if at all, 
and are sometimes angular, and can therefore be referred only toa 
source close at hand. It seems reasonable to conclude, on these several 
grounds, that the trap fragments in the sandstones that rest on the trap 
sheet of Saltonstall Mountain may be accepted as giving indication that 
the sheet had been formed before the deposition of the sandstone above 
it. The action of waves and tidal currents on the scoriaceous, irregular, 
and fragmental surface of a lava flow would be entirely competent to 
detach and transport relatively coarse pieces of the lava from more to 
less exposed situations, and mingle them with fine sands derived from 
more distant sources; and this process might continue with decreasing 
activity until the last. remaining knobs of lava were buried under the 
growing sandstone cover. This interpretation is the only one that ap- 
pears consistent with the facts here noted. The sandstone lying on the 
back of this trap sheet is distinctly harder than is common in the region, 
and our first impression was that its hardness was due to baking, and 
that the trap sheet was intrusive ; but this is not in the least borne out 
by more careful study. The hardness of the sandstone is due to 
cementation by infiltrating calcite in chief part, and not at all to 
change from the ordinary structure of sandstone. The sandstone on 
1 The Copper-Bearing Rocks of Lake Superior, Monogr. V., U.S.G.S., 1883, 
pp. 79, 1389, 140. 
