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Show 46 FLINT. NoTE XIX. · d ith thofe of vegetables, thefe flints bodies differs but little from ~ach other compar~ wd At the fame time as they cool- . 1 ·. 1 than thofe above ment10ne · veda ?1n lceofsn cwen t1t 1~~C~~ 1r pch oeoreu;~ike the Scot's-pebbles, they often polfef~ faint rings of colours, b k · eonchoide forms like them· . ami al.w~ys rcfah IQ d .n· of nodules of flint in chalk-beds is countenanced from Tlus Idea o t e pro u..:.uous . h' h . I ·ch ·enerally appears as thcfc flints become decompofed by the air; w IC the tr~~ w H 'thgthe 'tron in their compofition reduces it from a vitrefcent fl:ate to that of by Ullltlllg WI · . halk-calx and thus ren d ers I. t V·IrII: bl c. And fecondly , by there bemg no ap.p earanceh Ill c h' h b ds' of a fl:ring or pipe of filiceous matter conneel:ing one nodul~ ':"lth anot er, ~, IC t~ua have happened if the fili ceous matter, or its acid, had been tnjetl:ed from Without d. t the 1· dea of Dr Hutton. And thirdly, becaufe many of them have very accor mg o · ·' 1 b ~ J large cavities at their centres, whic~ iliould not have happened hau t 1ey een orme b the inje8ion of a material from Without. . y When iliells or chalk are thus converted from calcareous to filtceous 1~atter by tl~e fl clb of the animal , the new flint being heavier than the fhell or chalk occupl~s.lefs f~a~c than the maten·a 1s 1· t was pro d u ced from·, this is the taufe of frequent cav1t1es w1th111 them, where the whole mafs has not been completely fufed and preffed togethe.r. In Derbyiliire there are maffes of coralloid a~d othe: ~ells which h.ave become filt ceous,_ and are thus left with large vacuities fomettmes w1th1n and fomet1mcs on the ~utfide of t he rem<l·l n·m g 1c 0r m of the 111\1. ell , like the Fren•c h millfl:ones, • and I fuppo• fe m• tght ferve the fame purpofe; the gravel of the Derwent IS full of fpec1mens of th1s k1nd. S.m ce Tl'l'·l tl·n g tl 1e a hove I have received a very ingenious. account of. chalk-beds from Dr. MEN ISH of Chelmsford. He diftinguifhes chalk-beds mto three kt.nds; fu ch as have been raifed from the fea with little riifl:urbance of their fhata, as the cldfs of Dover an3 Margate, which he terms intire chalk. Another fl:ate of chalk is where it has fuffercd much derangement, as the banks of the Thames at Gravcfend and Dart:ord. And a third fl:ate where fragments of chalk have been rounded by water, wht ch he ~e rn_1 s alluvial chalk. In the firfl: of thefe fituations of chalk he obferves, that tb.e fltnt l1es Ill fl:rata horizontally, generally in difl:intl: nodules, but that he has ob ferved two in llances of folid plates or fl:rata of flint, from an inch to two inches in thicknefs, interpofed -between the chalk beds; one ofthcfe is in a chalk-bank by the road fide at Berkhamfl:ead, the other in a bank on the road from Chatham leading to Canterbmy. Dr. Meni!h has further obferved, that many of the echini are crufhed in their form, and yet filled with flint, which has taken the form of the crufhed fh ell, and that though man~ flint nodules are hollow, yet that in fome echini the filiceum feems to have enlarged, as It paffed from a fluid to a {i)] id fl:ate, as it fwells out in a protuberance at the mouth and anus of the fhell, and that though thefe iliells are fo filled with flint yet that in many places the fh ell itfdf remains calcareous. Thefe fl:rata of nodules and plates of flint fcem to countenance their origin from the fle01 of a fl:ratnm of animals which perifhed by fome natural \ iolencc, and were buried in their {hells. •1 ~ 1 I FLIN T. 47 7· AN":T ES OF SILICEOUS SAND. ·In many rocks of f.licenus fand the particles retain their angular form, and in fame beds of loo(e fand, of which there is one of confiderable purity a few yards beneath the · m:nl at Normington about a mile fouth of Derby. Other filiccous fands have had their a_ngles ro11ndcd otT, like the pebbles in gravel-beds. Thefe feem to owe their globular form to two c:mfes; one to the1r attrition againfl: each other, when they may for centuries have l.!in at the bottom of the fca, or of rivers; where they may have ·been progreffively accumulated, and thus progreffively at the fame time rubbed upon each other by the dafhing of the water, and where they would be more eafily rolled over each other by th~ir g ravity being fo much leis than in air. This is evidently now going on in the nver Derwent, for though there are no limdl:one rocks for ten or fifteen miles above Derby, yet a great part of the river-gravel at Derbyconfifl:s of limefl:one nodules, whofe angles are quite worn off in their defcent down the fl:ream. There is however another caufe which mufl: have contributed to round the angles both of calcareous and filiceous fragments; and that is, their folubility in water; calcareous earth is perpetually found fufpended in the waters which pafs over it· and the earth o~ flints was obferved by Bergman to be contained in water in the pro;orti.on of ~ne gra111 to a gallon. Kirwan's Mineralogy, p. 107. In boiling water, however, it IS foluble in much greater proportion, as appears from the filiceous earth fublimed in the difl:illation of fluor acid in glafs veffels; and from the bafons of calcedony which furrounded the jets of hot water near mount Hecla in Iceland. Troil on Iceland. It is probable mofl: filiceous fands or pebbles have at fome ages of the world been lono- • 0 expoled to aqueous fl:eams raifed by fubterranean fires. And if fragments of !lone were long immerfed in a fluid menfl:ruum, their angular parts would be firfl:diffolved, on account of their greater furface. Many beds of filiceous gravel are cemented together by a filiceous cement, and are called breccia; as the plumb-pudding fl:ones ofHartiordfhire, and the walls of a fubterraneous temple excavated by Mr. Curzon, at Hagley near Rugely in Stalfordfhire; thefe may have been expofed to great heat as they were immerfed in water; which water under g~eat preffure of fuperincumbent materials may have been rendered red-hot, as in Papin's d1gefier; and have thus poffefled powers of folution with which we are unacquainted. • 8. BASAL TES AND GRANITES. Another fuurce of filiceous fl:oncs is from the granite, or bafaltes, or porphyries, which are of different hard neff':!s according to the materials of their compofition, or to the fire they have undergone; fu ch are the Hones of Arthur's-hill near Edinburgh, of the Giant's Caufway in Ireland, and ofCharnwood Fore/1 in Leicefieriliire; the uppermofl: firatum of which Jail fecms to have been cracked either by its elevation, or by its hafiily cooling after ignition by the contaCt of dews or fnows, and thui breaks into angular fragments, fuch as the flreets of London are paved with; or have had their angles rounded br |