158 Geological Society : — 



A chemical analysis has been made with the greatest care r 

 according to the methods advocated by Hillebrand and the author, 

 zirconia and the rare earths being especially looked for. The main 

 results of the analysis confirm those of Makins, but several new 

 points of interest have presented themselves. The outstanding 

 features of the rock appear to be the high percentages of silica, 

 ferric oxide, and soda, and the low percentages of alumina, ferrous 

 oxide, magnesia, lime, and potash. The special interest of the new 

 analysis, however, lies in the detection of zirconia and cerium oxide 

 in unexpectedly large amounts : the percentage of cerium oxide 

 being larger than that from any known igneous rock, with the 

 exception of the nepheline-syenite from Almunge in Sweden. 



The author has calculated the norm from the old and the new 

 analyses, and finds that the rock falls into the subrang rockallose 

 Avith the general symbol III. 3. 1. 5. He notes, as indicating the 

 exceptional character of the rock, that these analyses are the only 

 representatives of the subrang rockallose among the 8000 

 analyses of igneous rocks that he has now collected. 



The author concludes with a discussion of the affinities of 

 rockallite, and a consideration of the probable chemical composition 

 of the segirite and acmite. He clearly proves that the zirconia 

 and cerium oxide enter into the composition of the pyroxenes. 



April 8th.— Dr. A. Smith Woodward, FJt.S., President, 

 in the Chair. 



The following communications were read : — 



1. ' The Evolution of the Essex River- System, and its Relation 

 to that of the Midlands.' By John Walter Gregory, D.Sc, F.R.S., 

 F.G.S., Professor of Greology in the University of Glasgow. 



The post-Eocene geology of Essex must be learnt from its 

 gravels and their non-local constituents. In the absence of any 

 rock which affords a certain proof of its route, the effort was made 

 to determine the direction of transport by tracing the variations in 

 the proportions and size of the non-local constituents ; this test 

 shows that the quartzites and felsites came from the north-west, 

 and the Lower Greensand cherts from the south and south-east. 



I. The gravels may be classified as follows :— 



(1) The oldest series. The Brentwood group, which 

 consists of redeposited Bagshot Beds and of local materials only. 



(2) The Danbury Gravel, which was deposited before the 

 arrival of the felsites, and at the beginning of the arrival of the 

 Lower Greensand cherts. 



(3) The Braintree Gravel, which is largely composed of 

 quartzitic drift, with abundant Lower Greensand cherts and some 

 felsites that were probably derived from the Lower Greensand 

 conglomerates north-west of Essex. 



(4) and (5) Glacial and post-Glacial gravels. 



