MUSEUM OF COMPAKATIVE ZOOLOGY. 65 



Below the cliflf, a few outcrops of sandstone or conglomerate may be 

 found. These data for a cross-section are less complete than those col- 

 lected on the previous day, but they suffice for a rough diagram added 

 in the margin of Fig. 3. 



On the supposition that the whole valley consists of an unbroken 

 monocline, the two sections now constructed can be placed in their 

 proper relative positions by means of a map, which would result in show- 

 ing that the second belongs about four and a half miles westward across 

 the strike from the first, and therefore its beds stand about five thou- 

 sand feet below those of Lamentation. But, if not already noticed, 

 attention will soon be called to the similarity between the two sections; 

 and the question then arises, how can this be best explained. It may 

 be the result of similar processes repeated in a given order, whereby 

 similar sequences of beds were deposited at different times ; or it may 

 be the result of one or more strike-faults, by which portions of a single 

 sequence of beds are brought to the surface in different places. It re- 

 mains to decide between these two alternatives. 



A general consideration of the problem will make it evident that the 

 explanation by repetition of similar processes becomes less likely, and 

 that the explanation by faulting becomes more likely, with the increase 

 in the number of beds in the repeated series ; with the lack of genetic 

 relation among the members of the repeated series ; and with the in- 

 crease in the number of times that such repetitions occur. It should 

 also be noted that the two explanations are not mutually inconsistent ; 

 both might apply in a single field. 



The drift covers so much of the surface that detailed sections cannot 

 be constructed. The stratified beds are, on the whole, so much alike, 

 that the precise identification of equivalent beds by agreement in com- 

 position is impossible. While it may yet be found that fossils will serve 

 as a guide to the recurrence of repeated outcrops, this means of identifi- 

 cation cannot at the present time be applied in the region we are exam- 

 ining. All that can be done is to make the best of imperfect evidence. 



We may first examine the argument based on the number of members 

 in the repeated series. The completed sequence of visible beds is : con- 

 glomerate and sandstone ; trap of moderate thickness ; thin limestone ; 

 shaly sandstone ; heavy sheet of trap ; more shaly sandstone ; a thin- 

 ner sheet of trap ; and, finally, more sandstone. It may be conceivable 

 that this sequence of beds was independently made at two different 

 times ; but it is not at all likely that so considerable an agreement 

 should be the result of the accidental repetition of a sequence of deposits. 



VOL. XVI. — NO. 4. 5 



