64 ME. S. S. BUCKMAN ON [vol. lxxvi, 



It is obvious from this diagram that only the species A and B 

 can be, not necessarily of the same date, but lying side by side in 

 the same deposit, by no means an identical proposition (see p. 85). 

 If C and D were in like condition, then section 3 should disclose C. 



If the line of railway be approximately level, then with the same 

 records a syncline of strata will be exposed. If the two middle 

 records changed places respectively with those outside, then an 

 anticline will be shown. 



If, however, the record presents itself as in fig. 2, then there are 



Fig. 2. — Supposititious faunal analysis. 

 1. 2. 3. 4. 



E 

 D I) 



C C 



B 

 A A 



lacunae in 1 & 3, and it is necessary to consider the various factors 

 which may produce such gaps. These are : — 



I. Stratal failure due to penecontemporaneous erosion, or to lack of 



deposit — depositional failure. 

 II. Faunal failure— due either to I, or to 



III. Dispersal failure — limited horizontal range of fauna. 



IV. Exposure failure — failure to uncover a portion of deposit at a 



certain place. 

 V. Collection failure — incomplete collection of available specimens. 

 VI. Arrangement failure — the order of species is plotted incorrectly. 

 VII. Nomenclature failure — especially lumping. 



There are also other phenomena to be considered — to account 

 for faunal lacunas, but they could not well be urged with regard to 

 contiguous exposures. They will be considered later (see p. 89). 



As regards I. stratal failure, it is now known from work in 

 the Jurassic rocks for many years, sufficiently illustrated in the 

 Society's Journal, that non-sequences due to penecontemporaneous 

 erosion are almost the rule and true sequences the exception ; that 

 the non-sequences have sometimes a horizontal range of several 

 miles, but that the effects of the erosion can be detected in a range 

 of a few yards — intermediate fauna and strata at one end of a 

 quarry or cutting having been wholly or partly removed from the 

 other. Here then is one reason for gaps in the faunal sequence : 

 such gaps should show themselves not as isolated lacunae, but 

 as following regular curves of thinning and thickening — lenticular 

 lacuna). Stratal failure due to depositional failure would be a 

 doubtful point in contiguous exposures, but would have to be con- 

 sidered in comparison of separated areas. 



