s 





296 



BULLETIN OF THE 



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'* there was a tilting of the strata in progress, before the final breaking 

 and ejections " (c, 421). This is also inconclusive; for the same relative 

 position of columns and strata obtains in the sheets that overflowed 

 on horizontal layers, the entire mass being tilted bodily afterwards. 

 We must therefore differ from the conclusion that the eruptions of the 

 trap were necessarily the " closing events of the sandstone period," or 

 that they took place in "a succeeding epoch" (c, 421). The overflow 

 sheets were certainly of earlier formation : the mtruded sheets may 

 have been so as welL 



Kussell states that the outbursts of trap occurred after the sedimen- 

 tary rocks had been consolidated and upheaved, and at the time when 

 the post-Triassio elevation culminated. His evidence for this view is 

 theoretical (c, 245, 251) and is seriously weakened by the occurrence of 

 overflows. The generalized section that he gives for the Palisade sheet 

 (df here copied, fig. 31) shows some down-branching dikes of proble- 

 matic occurrence. Cook classes all the traps as eruptive after the depo- 

 sition of the sandstones and shales (c, 32). 



The evidence which points to an early date for the eruption of the 

 intruded sheets is, first, the ragged line of contact with the sandstones 

 where they are broken across ; this, as has already been mentioned 

 under the dikes, indicates an eruption before the making of joint planes, 

 and consequently before the tilting; but it is not final or conclusive. 

 Second, the occurrence of intrusions chiefly on the outcrop side of the 

 sandstone belts, or, in other words, near the base of the formation ; the 

 only cause that I can suggest for such a limitation of position is that 

 these laccolitic intrusions could only form at a considerable depth, and 

 under considerable pressure, and were therefore placed near the bottom 

 of the sandstones before the latter were , tilted : if the intrusions had 

 taken place after the tilting, they might break out at one point as well 

 as another, and would not be likely to have so peculiar a restriction as 

 that above noted. Third, the intruded sheets have the same crescentic 

 form as the overflows; and this, as will be shown farther on, results 

 from folding after the eruptions : while it is possible that the intrusions 

 were guided into their curved line of outcrop by the gently folded strata, 

 it seems more probable that all were folded together. 



Eut, as stated above, this question is at present indeterminate; the 

 above suggestions may serve to counterbalance opinions on the other 

 side of the question, but not to settle the matter. It remains with a 

 number of other points, notably the monoclinal structure, open for 

 further observation. 



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