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amorphous variety of silica with conchoidal iracture. It was never 

 found crystalline or with mammillar)- or radiate structure. The first 

 thing which struck an observer in our chalk quarries and cuttings was 

 that the flint lay in layers. Dr. Mantell accounted for this by sup- 

 posing that streams of water highly charged with siliceous matter 

 streamed at intervals over the bottom of the sea, involving numerous 

 small organisms, which became fixed in the subtranslucent flint precipi- 

 tated from it. Although this theory was no longer held, no other 

 had, as far as he was aware, been propounded satisfactorily to account 

 for them. Personally he looked to the result of recent expeditions 

 to throw light on the subject, as showing us the conditions of tempera- 

 ture, depth of water, &c., which were more favourable to the develop- 

 ment of siliceous organisms than of calcareous ones. 



Professor Thompson says, in the paper from which he before 

 quoted, " On the 4th of February the tow -net dragging a few fathoms 

 below the surface, came up nearly filled with a pale yellow gelatinous 

 mass. This was found to consist entirely of diatoms." When full 

 accounts of " The Challenger" and Arctic expeditions were published, 

 they would probably be better able to theorize on these flint-layers. 

 But now as to the origin of those flints and the singular phenomenon 

 they presented. We found them of almost all possible shapes, tabular, 

 spherical, pear-shaped, branched, solid, hollow, stratified. We found 

 the siliceous substance filling the tests of Echini ; solidified round 

 pieces of wood, shells, sponges, &c. 



From the splendid collection of flints which Brighton owed to the 

 industry and beneficence of Mr. Henry Willett, he had taken the 

 striking specimens before them to illustrate their growth in all its 

 singular and interesting phases. Time would not allow him to notice 

 one-hundredth part of all that was worthy their attention there. He 

 must confine himself to one or tw-o salient points in their structure. 

 Take the case of one of those solid spherical nodules which were not 

 uncommon. Under the microscope a section of flint often presented 

 to us a mass of siliceous or silicified organisms cemented by siliceous 

 matter. He quoted the paper of Professor Thompson to show the 

 power of sea-water to dissolve — and therefore to hold in solution — 

 carbonate of lime and silex. They would remember also his account 

 of those nodules of manganese. Had these particles of silica a like 

 poAver of forming nodules by aggregation ? "I am reminded," says 

 Mr. J, E. Taylor (Ston; of a Piece of Chalk), " of the way in which the 



