176 



MR. JUKES-BPvOWNE AIS^D PROF. HARRISON 



Grant give, on the other hand, a decided reaction with acid, and 

 thin sections under the microscope show that foraminifera and 

 calcareous particles constitute a considerable part of their mass. 



By treating a thin slice with acid the result was very similar 

 to that obtained in the case of the Cleiand Hill rock ; the slice 

 did not break down, but much of the calcareous element was re- 

 moved, and a thin section of the remainder shows a groundmass of 

 minute particles of rather irregular shape. These appear to be frag- 

 ments of the organic constituents of the rock soldered together by a 

 cement of glassy material which, as before, has penetrated the 

 tubules and the interspaces of the foraminiferal cells. Another 

 part of the slice easily disintegrated in a heated solution of caustif 

 potash. 



Thus it would seera that the cementing material of the nodules 

 is of the nature of colloid silica, and that they are essentially 

 siliceous concretions, but there is no globular colloid silica like that 

 which so often accompanies the segregation of silica in the English 

 Cretaceous rocks. 



(3) Calcified Earths. — A sample of a dull yellowish and rather 

 hard rock ten feet above the base of the series at Bissex Hill was 

 sent to Miss Raisin, who reports as follows : — 



This rock is composed partly of siliceous organisms, partly of 

 crystalline calcite, with a very few depolarizing chips of minerals, 

 within an indefinite, amorphous groundmass. The calcite has no 

 connexion that I could observe with organisms, except a chance 

 adhesion to a radiolarian test; and it is almost entirely in the form 

 of rhombohedral crystals.^ These often enclose darker central 

 rhombohedra which have apparently been formed by a cessation of 

 the crystalline growth and the deposit of a dusty layer or a film of 

 sulphide of iron over the surface before the formation of the calcite 

 recommenced. Occasionally a larger crystal has a small projecting 

 rhomb appearing like an accretion. The forms of the calcite and 

 the indication of its intermittent growth suggest quiet, undisturbed 

 formation in situ from independent centres, but it is difficult to say 

 whether they were formed during the deposition or during the 

 subsequent consolidation of the rock. The mineral chips are very 

 few and are all angular (averaging '02 mm. in diameter) ; they 

 include a few broken fragments of brownish volcanic glass, some 

 felspar, and probably mica. They give but little clue to the mode 

 of origin of the rock, but on the whole it is most probable that they 

 were transported through the air to a sea on the floor of which 

 radiolarian tests were accumulating. 



(1) Bissex Hill, basal bed at the north end of the outlier. 



(2) do. do. grey limestone in the road about 60 feet from 



the top ot the hill (S.W. side). 



(3) do. do. hy the roadside on the N.E. side of the hill. 



These were examined by Mr. Hill. They are all hard, bluish- 



^ Compare the structure of the ' Coral (?) Mud, Codrington College,' de- 

 scribed by Mr. Hill in his i^ppendix to our first paper, p. 247, vol. xhii. of 

 this Jonrnal. Possibly the calcite grains in this mud were derived from the 

 erosion of calcified Oceanic Deposits, and not from the detrition of coral-reels. 



