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Show 42 FLINT. NoTE XIX. t. SILICEOUS TREESi I f. h r. n many o t c1e 1r an d -r o· ks a1·e found the impreilions of vegetable roots, which fecm l; • , to have been tnt c mo fl. unc ha ngca bl c p·•l rts of the plant, as fh clls and f. hark s. teeth arc found in chalk-beds from their bei ng the mofl: unchangeable parts of t_he . an_nn~l. ~-11 other infl:allccs the wood itfel f is penetrated, and whole trees converted 1~t~ fltnt, fpc~tmcns of which I have by me, from near Coventry, and from a gravel-ptt tn Shropfhlrc ncar Child' i\rchal in the road to Drayton. Other polifh ed fpecimens of :ege_tablc flints abound in the cabine ts of the curious, which evidently fhew the co~centnc ctrcle of woody fibres, and their interfl.ices filled with whiter filiceous ~atter, w1th the branching off of the knots when cut horizontally, and the parallel lmes of wood when _cut longitudinally. with uncommon beauty and variety. Of the:e I poffefs fome beaut1ful fpecimens, which were prefented to me by the Earl of U xbndge. . The colours of thefe filiceous vegetables are generally brown, from t~e tron, I fuppofe, or manganetre , w h't c h 1'n dLtced them to cryflallize or to fufe more eaftly. Some of the cracks of the wood in drying arc filled with white flint or calce~lon~, and ot_hers of them remain hollow, lined with innumerable fmall cryfl.als tinged wtth tron, whtch I fuppofc had a {hare in converting their calcareous matter into liliceous cryflals, becaufe the cryfials called Peak-diamonds areal ways found b edd c ~ in. an ochreous earth; and thofe called Briflol-flones are fituated on limefl:one coloured With JrOn. Mr. F. French prefented me with a congeries of filiceous cryHals, which he gathered on the crater (as he fuppofes) of an extinguifhed volcano at Cromach Water in Cumberland. The cryf1:als are about an inch high in the fhape of dogtooth or calcareous fpar, covered with a da:k fe~rugin~us matter. The bed on which they ref!: is about an inch in thicknefs, and ts Hatned with iron on its under-furface. This curious foflil !hews the tranfmutation of calcareous earth jnto filiccous, as much as the filiceous fhells which abound in the cabinets of the curious. There may fome time be difcovered in this age of fcience, a method of thus impregnating wood with liquid flint, which would produce pillars for the fupport, and tiles for the covering of houfes, which would be uninflammable and endure as long as the earth beneath them. That fome ftliceous produCl:ions have been in a fluid flate without much heat at the time of their formation appears from the vegetable flints above defcribed not having quite loflthcir organized appearance; from £hells, and coralloids, and entrochi being converted into flint without loling their form; from the bafon of calcedony round Giefar in Iceland; and from the experiment of Mr. Bergman, who obtained thirteen regular formed cryflals by fuffering the powder of quartz to remaiR in a veffel with fluor acid for two years; thefe cryfl:als were about the fize of fmall peas, and were not fo hard as quartz. Opufc. de Terra Silicea, p. 33· Mr. Achard procured both calcareous and filiceous cryflals, one from calcareous earth, and the other from the earth of alum, both diffolved in water impregnated with fixed air; the water filtrating very flowly through a porous bottom of baked clay. See Journal de Phyflque, for January, 1778. NoTE XIX. FLINT. 43 3· AGATES, ONYXES, SCOTS-PEBBLES T n fmall cavities of thcfe fa11d-rocks I am informed the beaut1· {'ul r1· d 1 • ' ' I' 11 1Ceot1sno u esare found whtch are called Scot's-peubles · and which on be'1ng ct1t · d·rr d' .n· ' Jn trrerent Jrecuons take the names of agates, onyxes, fardonyxes, &c. according to the colours of th 1· fl 1 · l 1 . . e mes or nrata w llC 1 t 1ey ex.lubtt. Some of th_e nodules are hollow and filled with cryflals, others have a nucleus of lefs compact fiiJceous matter which is gene -11 h' fi d d . I . ra y w Jte, ur-rou. n e wtt 1 many concentnc flrata coloured with iron, and other alternate firata of white ~gate or calcedony, fometimes to the number of thirty. . I thtnk thefe nodules bear evident marks of their having been in perfect fufion by et ther he~t alone, or by water and he~t, u~der great preffi.1re, according to the ingenious theor~ of Dr. Hut_ton; but I do not Jmagme, that they were injected into cavities from matcn~ls from Without, but that fome vegetables or parts of vegetables contaming more ll:on or manganefe than ~the~s, facilitated the complete fufion, thus defiroying the vcfi1ges of vegetable orgamzat10n, which were confipicuous 1 · 11 th r.1· . e 11 1ceous trees <Jbove mentt~n~d. Some of thefe ~odul~s being hollow and lined with cryfials, and others cont.llnmg a nucleus _of whtte fl_li:eou_s matter of a Ioofer tex1ure, fhew they were compofed of the matcnals then ex1fimg 111 the cavity; which confifl.ing before of loofe fand, mufl: take up lefs fpace when fufed into a folid mafs. Thefe filiceous nodules refemble the nodules of iron-Hone mentioned in note on Canto II. I. 183, in_ re~pect to their potleffing a great number of concentric fpheres coloured generally With ~ron,. but they differ in this circumfl:ance, that the concentric fpheres generally obey the form of the external crufl:, and i11 th eir not poffeffing a chalybeate nu~leus. The fiala cr ites formed on the roofs of caverns are often coloured in conccntnc firata, by th eir coats being fpread over each other at different times. and f<)me of them, a~ the cupreous ones, poffefs great beauty from this formation; b'ut as tl~efe are neceifar~ly more or lefs of a cylindrical or conic form, the nodules or globular flints above defcnbcd cannot have b~en confl:rucrcd in this manner. To what law of ~ature then is to be referred the produClion of fuch numerous concentric fpheres r 1 lulp ct to the law of con gela tion. When fait and water ar~ expofed to fevere fr ofly air, the fait is f.tid to be precipitated as the water freezes; that Js, as the hea t, in which it was diffolvcd, is withdrawn. where the cxperim~nt is tried in a bowl or- bafon, this may be true~ as the furface freez,es firfi, and tl~t: fait IS found at the bottom. But in a fluid expofed in a thin ph ia l, I found by <:x penment, that the extraneous matter previou(Jy diffolved by the heat in the mixture \Vas not fimply fet at liberty to fubfide, but was detruded or pufhed backward as the ice· w_as produced. The experiment was thi s : about two ounces of a folution of blue vitn ol_wcre acc~dcntally froz en in a thin phial, thegl afs was cracked mJ fallen to pieces, Jhe ICe was dtffolvecl, and I found a pillar of blue vitriol !landing erect on the bottom of the b'roken bottle. Nor is this power of congelation more extra ordina ry than that b . . ' Y Its powerful and fuddcn expanGon it fh otlld butfl iron {]Jells anJ coC'horns_, or throw F z |
