256 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 16, 



basin-shaped depression in the hard chalk, measuring rather more 

 than a foot across ; the slight depth of this cavity compared with its 

 diameter, showed that it was the cast of the lower portion of the 

 boulder. The original form of the boulder may be also shown 

 by fitting on the larger fragments to the remaining principal mass ; 

 it will then be clearly seen that it is a rounded water-worn block, 

 just such as may be met with by thousands on any coast-line of our 

 own seas, which is composed of rocks of* like mineral character, as 

 about the Scilly Isles, the Channel Islands, or those of Brittany. 



Character and composition of the Boulder, and of the associated 

 Sand and Greenstone. — The Purley boulder is a crystalline granitoid 

 rock, having its components very uniformly distributed : those now 

 remaining are quartz and felspar, from which, perhaps, some other 

 mineral has been removed ; what this may have been will be con- 

 sidered in the sequel. Whether the rock-mass here in question had 

 or had not undergone such a change before it was imbedded, can- 

 not now be determined with any certainty. It will, however, be 

 observed that the Purley block, which is by far the largest mass 

 which as yet has been found imbedded in the Chalk, and which 

 therefore, as far as size is concerned, would be well fitted to resist 

 the process of decomposition and the removal of one component, 

 presents a cellular character throughout, whereas the smallest pebbles 

 and fragments of other granitoid rocks exhibit no such structure. 



On showing a fragment of the large Purley block to Mr. D. Forbes, 

 he thought that he recognized it as corresponding in composition and 

 external characters with a peculiar granitic rock which was intro- 

 duced into the Scandinavian sedimentary series about the later palaeo- 

 zoic period. On the whole, therefore, I am disposed to consider 

 that the rock was in precisely the condition in which it is now seen 

 at the time it was imbedded, and that, if it has experienced any de- 

 composition, it was previously to its removal from its original site. 



That all the extraneous rocks which had been met with in the 

 Chalk should have occurred as isolated fragments, was somewhat 

 remarkable ; and the only case of an approach to anything like 

 definite relative position amongst them was the discovery of several 

 about the same time at Houghton, in the same pit, and whilst the 

 men were at work on the same bed ; so that the fragments there 

 may have been scattered about at the same time : but in the case of the 

 Purley boulder the case was very different. I will first refer to Mr. 

 Simmonds' account, quoted above, p. 254 ; he notices the occurrence 

 of sandy granitic matter, and fragments of decomposed greenstone, 

 associated with the boulder. 



When I subsequently visited the spot, the Chalk-rock had not been 

 so widely disturbed but that I was enabled to observe what offers a 

 direct corroboration of the previous statement. The large boulder 

 had left a clean and distinct cast of a part of its rounded surface 

 in the compact Chalk. Some of the larger fragments of greenstone 

 according to the description of the pitmen, "had been stuck about 

 it, and over it ;" but on looking closely, I saw that there was a mass 

 of sand extending from the level of the cast of the boulder and 

 beneath the solid chalk. Instead of removing the sand, I got the 



