Geological Society of London. 221 



leEiPOie-Ts j^isTT) i=iaocEE3Diisrc3-s. 



Geological Society op London. — I. Marcli lOth, 1869. — Papers 

 read : 1. " On the Origin of the Northampton Sand." By John W. 

 Judd, Esq., F.G.S., of the Geological Survey of England. 



This paper was an attempt to base on the study of a rock, both in 

 in the field and the laboratory, a complete and consistent theory of 

 the conditions of its original deposition, and of the sequence and 

 causes of its various metamorphoses. 



The Northampton Sand was described as consisting of various 

 strata, usually of an arenaceous character, which frequently pass, 

 both vertically and horizontally, into a ferruginous rock, the well- 

 known Northamptonshire ore. 



The different features presented by the formation in various 

 localities were then indicated ; and the lithological, microscopical 

 and chemical characters of its constituent rocks described at length. 



These characters were shown to point to the conclusion that the 

 beds were accumulated in a delta of one or more great rivers. 



Arguments were then adduced in opposition to the theory of the 

 formation of ironstones by direct deposition, and in favour of the 

 hypothesis that the Northamptonshire ore consisted of beds of sand 

 altered by the percolation through them of water containing carbo- 

 nate of iron. 



The cause of the re-distribution of the iron in the rock was then 

 discussed, and, in opposition to the views of Mr. Maw, who has 

 referred the phenomena in question to " segregation," they were all 

 shown to be easily capable of explanation on well-known chemical 

 principles, and to be due to the action of atmospheric water finding 

 access to the rock by its joints and fissures. 



The paper concluded with a sketch of what was inferred to be the 

 history of the rock from its accumulation to the present time, and 

 some remarks on the varied and important effects of water when 

 acting under different conditions on rocks. 



Discussion. — Mr. Maw a^eed on the whole with the author as to the chemical 

 changes which had taken place in these beds. He had little doubt of the carbonate 

 of iron having replaced carbonate of lime, as had been originally suggested by Mr. 

 Sorby, and of there having at one time been a much larger proportion of protoxide 

 in the beds. He did not, however, think that the banded arrangement of the prot- 

 oxide of iron could be accounted for on any simple chemical theory. He had found 

 by analysis that the proportion of iron in the dark and light areas of variegated rocks 

 bore no direct proportion to each other. In some cases the whole of the iron in the 

 lighter portions had been exhausted. The excess of phosphoric acid in some 

 patches could, he conceived, be only accounted for by segregational action. The 

 thickness also of the environing band was always proportional to the area enclosed, 

 and not by any means uniform round blocks of diiferent sizes. It therefore seemed 

 to be due to segregation from the included mass from which a certain portion of the 

 iron had been withdrawn. 



Mr D. Forbes objected to the name of Northampton Sands as applied to rocks so 

 slightly arenaceous. He thought the phosphate of iron present might be principally 

 due to the organic remains in the original limestone, which had been replaced by the 

 carbonate of iron. He remarked that in many of the blue ironstones there was no 



