﻿lxviii PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [May 1905, 



of graptolites as nuclei. These nodules are probably not absent 

 from the other deposits mentioned, but in any case are much rarer 

 therein. In Wenlock times, then, some physical conditions existed 

 which caused the accumulation of strata of a type adapted to the 

 formation of elliptical concretions within them. 



"We have no indication as to the nature of these conditions ; the 

 point of importance to us is that, owing to some cause or causes, 

 a comparatively-inconspicuous character of certain rocks is found, 

 as the result of observation, to be of greater value as a 

 time-index than other characters which are more obvious. 



It is only by observation that the relative values of different 

 lithological characters can be gauged, and accordingly very detailed 

 observations of these characters are important. Apart from the 

 composition of the rocks, information of use for classificatory 

 purposes may sometimes be obtained by noting such features as the 

 closeness of the lamination of muds, the nature of the minor jointing 

 of the beds, and the mode of preservation of the included organisms. 

 For instance, the muddy rocks of many parts of England and Wales 

 are often found to be more finely laminated among the Silurian 

 rocks than among those of Ordovician age ; the peculiar nature of 

 the jointing of the Menevian rocks of Wales distinguishes them from 

 argillaceous rocks of Lingula-Ylag age, which are otherwise some- 

 what similar ; and, in the Cambridge district, the preservation of the 

 ammonites of the Ampthill Clay in phosphate enables us to dis- 

 tinguish that formation from the Oxford Clay of the same region, 

 where the ammonites are preserved in pyrites. 



The more detailed study of the petrography of the sediments, 

 which is now being pursued after a period of comparative neglect, 

 may throw unexpected light on the value of lithological characters 

 for purposes of time-classification. 



(3) Accumulation of Contemporaneous Volcanic Materials. 



The occurrence of the products of contemporaneous volcanic 

 action is, of course, useful locally in determining the age of 

 strata, although, owing to the frequent shifting of vents, it is 

 dangerous to use this evidence over very wide areas. For instance, 

 in many parts of the British Isles, the existence of much con- 

 temporaneous volcanic material in Ordovician times, and its absence 

 from the Silurian strata, furnishes a ready means of distinction ; 

 but the presence of Silurian volcanic rocks in the Tortworth area 



