Vol. 56.] CONSTITUENTS OF THE CRAG. 739 



localities, but these largely depend on varying degrees of impregna- 

 tion with iron. The iron-staining undoubtedly took place after the 

 beds were deposited, and differences in amount can be traced to the 

 flow of water containing iron in solution. Thus, it is found that 

 where the Crag rests upon a clayey foundation, as at Beaumont, the 

 line of parting has formed a channel along which water has flowed. 



In some cases, as is well seen in the pit near Beaumont Hall, the 

 lines of infiltration run across the bedding in wavy lines. It is 

 evident that water percolating through porous rock must trend 

 towards an outflow. When a line of flow has once been established, 

 most of the water will move through certain channels and, carrying 

 iron in solution, there will be a tendency to form pipes by the 

 deposition of iron-oxide in the interstices of the sand. 



Where pits have been for a long time unworked and a cliff of 

 Crag has been left exposed, the porous rock has served as a passage 

 for surface-water, which oozes out from the face of the section. 

 Here the staining can be directly associated with lines of flow. In 

 some instances the iron-oxide has cemented the sand and shells 

 together, and, where a part of the cliff has been overhanging, 

 ferruginous stalactites and stalagmites have formed. This is well 

 shown in the pit below the church at Chillesford. The stalactites 

 are hollow tubes, frequently branched, and water can now be seen 

 dripping through them. 



One case of infiltration is worthy of special mention. It occurs 

 at the base of the section at Beaumont Hall, where the Crag rests 

 directly upon the London Clay. The sand and shells are black, 

 and so thick is the encrusting material that no part of the 

 original fragments can be seen. On treating this material with 

 hydrochloric acid the calcareous interior is removed, but a hollow 

 cast is left composed of iron-oxide. The carbon dioxide, liberated 

 by the action of the acid on the shell, buoys up the cast so that it 

 floats on the surface of the liquid. 



Casts of shells are produced which often retain their characteristic 

 markings. Even perfect examples of Echinocyamus and polyzoa can 

 be obtained in this way, and some of the latter, such as Biflustra and 

 CeTlaria, show all the delicate features of the original organisms. 



The following analyses, made by Mr. C. C. Moore, F.I.C., of 

 Liverpool, show that the black encrusting material is slightly 

 phosphatic : — 



No. I refers to the black material from Beaumont ; 



No. II, given for comparison, is from the same pit, but the material was 



obtained from the interior of a large Fusus ; 

 No. Ill shows the composition of a lighter-coloured Crag from Oakley. 



I. 

 per cent. 

 Silica, etc. insoluble in hydrochloric acid 44'95 

 Soluble in hydrochloric acid: — 



Si0 2 trace 



Fe 2 3 572 



Mn 2 6 3 -21 



CaC0 3 4821 



MgO 08 



• PA 21 



II. 



III. 



per cent. 



per cent. 



41-58 



59-63 



none 



none 



7-73 



16-37 



trace 



trace 



49-82 



23-16 



•13 



trace 



trace 



trace 



3d2 





