part 1] 



JURASSIC CHRONOLOGY : LIAS. 



63 



excavation for a gasometer, lias in my opinion dealt with a fauna 

 in part from a still lower horizon, about Mercian 7 1 ; though 

 Mr. Richardson locates it all as oxynotum^ say early Deiran. 



The specimens were not collected in situ, except in one instance 

 — an almost impossible task with modern , methods of railway- 

 construction, — and hence it might he thought that the evidence 

 would not be good. But the range of the strata which Mr. Rich- 

 ardson has divided into seven or at the most eight hemerae cover, 

 according to my more detailed subdivisions, thirty-two hemerae, 

 and should afford a good test as to whether this more detailed 

 method of dating makes for greater precision in recording know- 

 ledge of local deposits. 



II. Faunal Analyses. 

 (a) Methods of Work. 



The problem presents itself in this way — given sections along a 

 line of country disclosing dissimilar faunas — in this case Ammonites 

 and certain Brachiopods are the subject of the enquiry, — find the 

 sequence of the deposits from the recorded faunas. It is obvious 

 that, if the various sections be placed in their geographical order — 



Fig. 1. — Supposititious failnal analysis. 



f. 2. 3. 



in the present case easy, as it is approximately a straight line of 

 country — , and if the species be correctly placed in sequence, a series 

 of regular curves without gaps must result from the plotting of 

 the faunal analyses according as the sections are through higher 

 and lower strata. To illustrate this, suppose that there are taken 

 four localities with five species represented by the letters A-E. 

 Then the above supposititious diagram may result (fig. 1) — 

 the curve Y represents the line of the railway, the other (Z) the line 

 of hill and vale ; and the places numbered are the cuttings ; in 

 between are hollows bridged by embankments or otherwise. 



II, 8, p. 271. 



2 XVIII, 1, p. 155. 



