Vol. 54.] 



AND THE CORALLINE CRAG. 



323 



Fig. 2. — Rod-like bodies in Coralline 

 Crag residues (after solution in hy- 

 drochloric acid). 



In some borings at 

 Gedgrave and Sudbourne, 

 to be described hereafter, 

 I found at the base of 

 the formation a bed, 

 12 inches or so thick, 

 resting on the London 

 Clay, which it resembles 

 in colour, consisting of 

 blue Crag which differs 

 from the shelly sands in 

 containing a larger pro- 

 portion of quartz-grains. 

 Analysed, it was proved 

 to contain : — 



No. ^6 \ 



( Carbonate 



of Lime Sihca 

 57-30 0/0 3510 «/o 



Alumina and 



Oxide of Iron 



3-80 0/0 



X 100 diameters. 



the iron being mainly in 



the unoxidized or ferrous 



state. 



Dr. Hinde observed in 



this blue Crag ' numerous 

 grains of a translucent mineral, probably a silicate of iron, allied 

 to glauconite ; they are mostly subangular, and polarize feebly 

 between crossed nicols.' No polyzoa were noticed,, but this absence 

 may be accidental, as Prestwich foand them at the base of the 

 Crag at Sutton. 



The consolidation of the ferruginous Crag and the dissolution of 

 the arragonite-shells seem to have taken place at a comparatively 

 remote period, that is, previously to the deposition of the Red Crag, 

 XLS both Sir Joseph Prestwich^ and Mr. P. F. KendalP found blocks 

 of the former in the latter deposit ; but the Coralline Crag has also 

 been subjected to the action of acidulated water in more recent 

 times. Mr. Sutton has analysed some dark red earth from the 

 cylindrical pipes, familiar to all students, which penetrate the Crag 

 from the surface downward, often to a considerable depth, and finds 

 that it contains the merest trace of calcium carbonate, as will be 

 jseen from the following analysis : — 



No. 7 



Carbonate of 





Oxide of 





Lime 



Sihca 



Iron 



Alumina 



0-56 0/0 



74-20 «/o 



11-40 «/o 



2-10 0/0 



^ Quart. Journ. Geol. Soc. vol. xxvii (1871) p. 341. 

 ^ Geol. Mag. 1883, p. 499. 



