320 Geological Society. 



This bed is separated by a well-defined line from another of red 

 gravel, about six feet thick, resting upon London clay. It is com- 

 posed rf rounded chalk flints and sand, and saurian vertebrae are 

 occasionally found in it j but no remains of Mammalia have been no- 

 ticed either in it or the superior bed. Mr. Spencer states that there 

 appears to be, in the whole of the deposit, a total absence of the 

 small rounded pebbles of Lickey quartz, which are plentiful on the 

 summits of the neighbouring hills of Highgate and Hampstead : and 

 in conclusion he suggests that the current of water which brought the 

 materials of the upper bed into their present situation flowed from 

 the north. 



March 25. — A paper was read, entitled " Remarks on the Struc- 

 ture of large Mineral Masses, and especially on the Chemical Changes 

 produced in the Aggregation of Stratified Rocks during different pe- 

 riods after their deposition j" by the Rev. Adam Sedgwick, F.G.S., 

 Woodward ian Professor in the University of Cambridge. 



§ 1. Introduction. 

 The first section of the paper is devoted to some general considera- 

 tions of the changes produced both by igneous and aqueous agents. 

 Changes of the former class may be effected in a comparatively short 

 period, and can sometimes be imitated in a laboratory. But changes 

 of the latter class have often been effected during indefinite periods of 

 time, and under circumstances which admit not of imitation. In such 

 cases it is by observation only, and not by direct experiment, that we 

 can hope to rise to a rational explanation of the phenomena. The 

 author then gives some examples of both kinds of change here con- 

 sidered. 



§ 2. Globular Concretionary Structure. 



The author remarks, that although this kind of structure, as seen 

 in aqueous rocks, can seldom be imitated, yet it may be explained, in 

 most cases, compatibly with the known modes of material action, and 

 the phenomena may be correctly classified. He then proceeds to 

 give examples of the structure in question. 



1. Chalk Flints. — They are posterior to the existence of the beds 

 in which they are found. The free siliceous matter of the formation 

 has not been uniformly diffused, but accumulated in distinct concre- 

 tions ; and therefore illustrates the principle contended for in the 

 paper. 



2. Globular Calciferous Grit, 8$c. — The author dwells at consider- 

 able length on the internal structure of calciferous grits, and explains 

 their chatoyant lustre by the independent crystallization of carbonate 

 of lime through definite spaces. He then points out several cases of 

 such rocks with a regular spheroidal structure, and with the lamina- 

 tions of original deposit passing, without interruption, through the 

 several spheroids ; and he infers from such phenomena that the glo- 

 bular structure was superinduced during the passage of the stratified 

 mass into a solid state. 



3. Globular Magnesian Limestone. — For a detailed account of this 

 structure, he refers to a former memoir in the Society's Transactions, 



