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 Keweenawau 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 valle}^ 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 traj) 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 to a 

 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 onh' 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 Tlie ropper-Bearing Rocks of Lake Superior, Mouogr. V., U. S. G. S., 1883, 

 lip 79, \Z0, 140. 



