HA RD WICKE'S SCIENCE- G OSS/F. 



229 



remarkable explanation ; viz., that rain-water charged 

 with carbonic acid soaks through the sands, dissolves 

 the chalk, and leaves behind only the flints and other 

 insoluble residue forming or including glauconite. 

 A thin bed of these glauconite-coated flints always 

 forms the junction between the Thanet sands and 

 chalk. 



On entering the side of the pit at Luton, the first 

 objects which attracted our attention were the regular 

 horizontal layers of flints so characteristic of the 

 upper chalk, pointing to.the abundance of silica in the 

 waters of the later cretaceous period. A short dis- 



be found to consist of irregular granules of limestone, 

 foraminifera in all stages of completeness, tiny ir- 

 regular granules and minute oval nodules of flint, 

 siliceous spicules of sponges in great profusion, minute 

 pclyzoa, tiny fragments and nodules of carbonate of 

 iron, etc., etc. 



It is therefore easy to imagine that water perco- 

 lating through the chalk would take up a large 

 quantity of this siliceous matter in its course, and 

 re-deposit it in a large horizontal joint, and thus 

 gradually form a great mass. 



As the flints are excavated they are thrown up by 



Fig. 137. — Ana^ichytes ovata, or "fairy loaf." a, base showing 

 position of mouth and anus. 



Fig. 139.— INIicraster. 



Fig. 140. — Galcrites albogalcrus. 



Fig. 138.— Natural flint cast of Ananchyte, showing 

 perforations (in relief) for ambulacral or sucking- 

 feet. 



Fig. 141. — Tetractinellid spicules (upper chalk, 

 Luton). 



Fig. 142. — Nat. size. Siphonia 

 showing internal radiated 

 structure. 



Fig. 143. — Parasmilia. 

 centralis. Primary 

 theca. 



tance from this pit is a remarkable layer, three 

 inches thick in one sheet, extending with scarcely a 

 break for about fifty yards, with a dip northwards of 

 about 10°. INIr. Gamble has traced this layer at two 

 other points about five hundred yards distant.* It 

 seems quite clear that this great tabular sheet must 

 have been produced subsequently to the laying down 

 of the upper chalk. 



If a small quantity of the chalk from this pit be 

 washed and examined with an inch objective, it will 



* Vide also p. 22, "Report London Geol. Field Class," 1S8S. 



the quarrymen into a huge heap. This 'we at once 

 attacked. The flints, roughly speaking, seemed to 

 be of three sizes — large irregular lumps, spongiform 

 or nodular pieces, and small smooth lumps. 



On breaking open some of them, most exquisite 

 linings of quartz crystals and chalcedony were brought 

 to light ; the crystals ranging from a most minute size 

 to a little over one-eighth of an inch across the base 

 of the pyramid. In many cases they were of beauti- 

 ful colours, apparently caused by being overlaid with 

 oxide of iron, etc. One such specimen possessed a 

 most gorgeous, greenish iridescence ; another closely 



