402 THE LOWER LIAS OE SEDBURT CLIEE. [Aug. I9O3, 



It may be hoped that the construction of range-diagrams dealing 

 only with the periods during which a common species was abundant 

 (and therefore independent of any small error in observation), will 

 be of use in testing the value of a series of ammonite-ages as 

 divisions of relative time. The errors to which such a series seems 

 a priori liable, are (1) irrationality or lack of proportion — that is, the 

 ratio of any two successive ranges is not the same in two different 

 localities ; and (2) acceleration or retardation of the ammonite- 

 ages, 1 when measured against the variation of longer-lived forms. 



Discussion. 



Mr. H. B. AVoodward said that the careful observations of the 

 Author could not fail to be of great service. With regard to 

 the extent of the Cotham Marble, it occurred not only over the 

 Radstock area but southward into Dorset. 



The Chairman (Sir Archibald Geikie) said that he would not 

 prolong the discussion, but would like to enter his protest against 

 the introduction of such a barbarism as ' range-graph ' into geo- 

 logical terminology. Men of science were sometimes censured for 

 their indifference to literary requirements and their love of a 

 cacophonous nomenclature, and geologists had to bear their full 

 share of this reproach. He hoped that the Author would find some 

 other term that would equally express his meaning, and give no 

 cause of offence to those who would like to keep the well of 

 English undefiled. 



which is crowded with them : here they occur as flattened, iron-stained casts, 

 but the absence of any recognizable trace of ribbing and the growth-rate, so 

 similar to tbat of the specimens from Watchett, make their identification 

 almost certain. This is further confirmed by comparing the specimens from 

 a similar horizon at Stoke Gifford, where they occur in the same abundance 

 and in a similar ferruginous matrix, but are somewhat better preserved. 



Although I was not fortunate enough to confirm the observation at the time 

 of our joint visit to Sedbury Cliff, Mr. Richardson had noted on a previous 

 occasion the occurrence of Ammonites Johnstoni in a bed lying well within the 

 range of Am. planorbis and in which I found undoubted specimens of the 

 latter form. Though I have never actually observed Am. planorbis and 

 Am. Johnstoni occurring together in the same bed, yet the early occurrence 

 of the latter at Sodbury goes far to confirm the accuracy of Mr. Richardson's 

 observation. There is no doubt, however, that, at Sedbury Cliff, as in the 

 whole of the area to the south and east, the zone of abundance of Ammonites 

 Johnstoni occurs in the beds above those containing Am. 'planorbis. 



1 The term ' age ' is used instead of ' hemera,' as simply implying the period 

 during which a species flourished at any locality, without for a moment sug- 

 gesting that this period is the same astronomical epoch at two different 

 localities. 



