532 Professor J. W. Gregory — The Danbury Gravels. 



Belgium, and this view received some support from Mr. Clement 

 lleid's adoption (1882, pp. 56-7) of the same source for the non-local 

 pebbles in the Forest Bed gravels. 



The solution of this problem may be attempted by the discovery of 

 the original home of the pebbles or by the study of their distribution 

 in Essex. The application of the former method to the quartzites 

 does not promise final results, for these rocks are so very durable 

 that they were widely scattered in Mesozoic and perhaps later 

 conglomerates, so that the determination of the original home of these 

 pebbles may give no indication as to the direction of their last 

 journey. 



The study of the quantitative distribution of the foreign pebbles in 

 Essex offers a more reliable method ; and it is available in all the 

 constituents of the gravels which were derived from outside Essex. 

 Accordingly, at intervals during many years past I have estimated 

 the percentage of the various pebbles in the chief Essex gravels. 

 The results are shown in the table on pp. 534-5. 



This table seems to me to prove the following conclusions : — 



1. That the quartzites entered Mid-Essex from the north-west. 

 For the quartzites and pre-Mesozoic cherts are most numerous and the 

 pebbles are largest in the north-west of Essex, and they diminish in 

 number and size to the south-east. Thus they occur to the extent 

 of from 20 to 25 per cent in Danbury, and over 40 per cent near 

 Great Dunmow ; but they number only 3 per cent and even less near 

 Burnham and Southminster. 



2. That the Lower Greensand chert came from Kent and 

 Surrey: for it is most abundant and occurs in largest pebbles in 

 Southern Essex; it occurs at low levels as far north as Braintree, 

 and even into Suffolk ; but in the high-level Danbury gravels it is 

 extremely scarce, and it is absent from North-Western Essex. That 

 this chert was not obtained from the Lower Greensand of Bedfordshire 

 or Cambridgeshire, which is not known to contain it, is further 

 shown by its absence from the gravels in those counties. 



3. That a series of felsites * (a name that maybe conveniently used 

 to cover the various igneous rocks referred to in Dr. Salter's memoir 

 as rhyolite) are widely distributed in North- Western Essex ; they are 

 absent from the Danbury gravels, but occur even to the south-east of 

 Danbuiy in the low-level gravels. According to the evidence 

 collected by Dr. Salter the introduction of these felsites was pre- 

 Glacial. 



4. That the Danbury gravels did not contain any material 

 introduced to Essex by ice. 



5. That the Danbury gravels contain abundant pebbles derived 

 from the Lower Eocene or Bagshot pebble-beds. The Danbury gravels 

 are therefore later than the Lower or Middle Eocene ; and the 

 absence of glacial constituents and their position show that they are 

 pre-Glacial. 



The best chance of the determination of a more precise age is given 

 by the distribution of the Lower Greensand cherts. They can only 



1 These felsites are probably derived from Lower Greensand conglomerates 

 in East Anglia or in the adjacent counties. 



