550 REPORT — 1882. 



portions of silicic acid and clay. Thus the great clay-deposits of the earth's crust 

 indicate a correspondingly large quantity of silica -which has been carried in solu- 

 tion into the sea. That sea-water contains but the merest trace of silica, proves 

 how effectually it is removed by sponges, radiolaria, and diatoms, in the formation 

 of their siliceous skeletons, and there is no other way in which solid silica can be 

 deposited in the open sea than through organic agency. Since modern calcareous 

 ooze always contains a certain percentage of siliceous skeletons, it has been con- 

 cluded that these were also present in the ancient ooze of the chalk ; since they 

 have disappeared from the chalk as it now exists, while flints have made their ap- 

 pearance, it has been suggested that the latter have originated by the transforma- 

 tion of the former. This conclusion has been confirmed by the observations of the 

 author, who, together with Zittel, has shown that the silica of siliceous organisms 

 has by no means the insolubility which has till recently been attributed to it ; and 

 next by researches of the author on incompletely formed flints, which have shown 

 that the flrst step in their formation is the replacement of a certain quantity of 

 chalk by silica, the cocco-liths, foramiuiferal and other calcareous shells becoming 

 siliceous pseudomorphs, which retain all the markings, even to the very finest, of the 

 original tests. A further quantity of silica is next deposited, filling up the chambers 

 oftheforaminifera and tlie interstices of the siliceous chalk; in this way a white 

 or grey flint is produced. If silica continues to penetrate the nodule and to be de- 

 posited in it, the grey flint loses its opacity and whiteness, and becomes converted 

 into black translucent flint. A nodule is seldom, however, uniformly black through- 

 out, certain portions remain in the previous grey-flint stage, forming light-coloured 

 spots and blotches on a dark ground ; if the white and black flint form alter- 

 nate layers, then we have the phenomenon of 'banded' flint. A specimen in 

 the Bristol Museum shows tlie formation of banded flint in an exceptionally clear 

 way ; the centre remains in the ' white ' stage, the exterior is black ; between the 

 two is a region of agate-like bands of black and wliite flhit, the white bands, 

 terminating at their ends as irregular capes or promontories, which project into 

 the black flint : while continuing their direction, are little outlying islands of white 

 flint, which exactlj' resemble the spots and blotches of an ordinary black nodule. 

 An examination of this specimen renders it clear that the nodule was originally 

 grey throughout, silica continuing to enter it from without converted the exterior 

 into black flint ; but the process having stopped short of completion the centre 

 remained unchanged, while between the centre and the exterior the intermediate 

 zone remains in an intermediate state as banded flint. Thus the grey, banded, and 

 black flizits — the distinction between which has long been recognised — are shown 

 to be but so many dift'erent stages of one and the same process. 



WUBXESBAY. AUGUST 30. 



The following Papers and Report were read : — 



1. Problems in tlie Geology of the Ohannel Islands. Bj tlie Rev. Edwin 



Hill, M.A., F.G.8. 



Attention is directed to the scanty information existing with reference to these 

 islands and the field of research thej' offer. McCulloch, Ansted, Liveing, have 

 written on them, but much remains unknown. 



The age of the rocks may be archsean, but requires fixing, and their relations 

 to surrounding areas have still to be made out. 



The stratigraphy of the separate islands has been worked out by Liveing, fully 

 for Sark, partially for other islands, but his results require confirmation and 

 extension. The mutual relations of Sark and Guernsey are given, but the rest are 

 entirely unknown. 



