SLATE. 



It may deferve notice, that as the felfpar is of a flelh-red 

 colour, it is impoffible for any obfervcr to confider them 

 as quartz veins. One of thefe large veins is diflocated, 

 and heaved feveral yards by a crofs courfe. Another of 

 thefe veins of granite, after proceeding vertically fome 

 diftance, fuddenly forms an angle, and continues in a 

 direftion nearly horizontal for feveral feet, with fchiltus, 

 jr Hate, both above and below it. This appearance moll 

 completely deltroys one of the theories fuggefted for the 

 explanation of fimilar veins at St. Michael's Mount ; -viz. 

 ttiat a ridge of projefting granite had been left, and flate 

 depofited afterwards on its fides." It is, indeed, quite 

 ■mpoffible to conceive that a narrow vein of granite (bould 

 !ioot up into empty fpace for a confidcrable dillance, and 

 tlicn atlume a zig-zag direction, unfupported by any (olid 

 fubftance on the fides. The flate-rock mult have cxilled 

 when the vein was formed, and from its refiitance have 

 .riven it the form above defcribed. Slate is one of the molt 

 abundant rocks in alpine dillrifts, though that variety 

 which fplits into thin durable laminae, forming good roof- 

 flate, is far from being fo common. The localities of com- 

 mon flate are fo numerous in mountainous diltritls, that it 

 were ufelefs to point them out. Roof-Hate, the fchifle 

 ardoife of the French, is found on the weftern fide of our 

 ifland in the counties of Cornwall and Devon, in various 

 parts of North Wales and Anglefea, on the north-welt 

 parts of Yorkfhire, near Ingleton, and in Swaledale, alfo 

 in the counties of Cumberland and Wellmoreland. It 

 occurs in a low range of mountains at Chamwood forelt, in 

 Leicetterlhire, near the centre of England. Several geo- 

 logifts have Hated that flate is found in Derbyfhire, which 

 is a miftake, as no rock fimilar to clay-flate or roof-flate 

 occurs in that county. The inhabitants of that part of 

 England make ufe of a micaceous fand-llone for roofing, 

 which they denominate flate. This ftone is found in Itrata 

 among the other llrata accompanying coal. Of the qua- 

 lity of the different kinds of Englifh roof-Hate, we Ihall 

 offer fome obfervations at the conclufion of this article. 

 Slate abounds in various parts of Scotland, and in the 

 county of Wicklow, and other mountainous parts of Ire- 

 land. France polfeffes many valuable beds of roof-flate, 

 near Laferriere in Normandy, and in the neighbourhood of 

 Angers. The lall is the moll important, as furnifhing 

 flate of the moll perfcdl quality, and its extent and pro- 

 digious thickiiefs make it regarded as inexhauftible. It is 

 further remarkable on account of the very fingular and 

 intereiling organic remains that occur between fome of the 

 lamina; of Hate. It is, perhaps, one of the moll valuable 

 rcpofitories of Hate at prcfent known. 



This bed extends for a fpace of two leagues, pafling 

 under the town of Angers, which is built, as well as covered, 

 with the Hate ; thofe blocks being employed in mafonry 

 which are the Icaft divi'fible. The quarries which are ac- 

 tually explored, are all in the fame line from well to eaft, 

 as well as the ancient pits, the bed of tiie befl roof-flite 

 rifing to the furfacc in this diredtion. Immediately under 

 the vegetable earth is found a brittle kind of flate, which, 

 for four or five feet deep, fplits into rhomboidal fragments. 



A little lower is wliat is called the building-llone, which 

 is a firm flate, but fcarccly divifible. Tiiis is employed 

 in the conltruftion of houfcs, after it has been fufficienlly 

 hardened by expofure to the air. At fourteen or fifteen 

 feet from the fnrface is found the good Hate, which has 

 been quarried to the perpendicular depth of about three 

 hundred feet ; the remaining thicknefs being unknown. 



As to the interior llrudlure of this great mals of Hate, 

 it is divided by many veins or feams of calcareous fpar and 



quartz, about two /eet thick, by fifteen cr fixteen in height : 

 thefe veins are parallel, and proceed regularly from well to 

 eaft, in a pofition rifing feventy degrees to the fouth ; they 

 are interfefted by other veins at intervals of a fimilar kind, 

 but whofe rife is feventy degrees north ; fo that when they 

 meet the former, they either form rhombs, or half rhombs, 

 which have been compared to the letter V, fome being 

 upright, and others reverfed. 



All the layers, or laminx, of the Hate, have a direftion 

 fimilar to thofe of the veins of quartz, which rife fouth 

 feventy degrees ; and when interfefted by veins that have an 

 oppofite inclination, the direftion of the Haty laminx is not 

 changed. The whole mafs is thus divided into immenfe 

 rhomboids, compofed of plates all parallel among them- 

 felves, and with two of the faces of the rhomboid. 



The Hate of Angers is extracted in blocks of a fixed 

 fize, which are divided into leaves for roofing-Hate. It is 

 betwixt thefe leaves that there are frequently found vef- 

 tiges of marine animals, and particularly pyritous imprcffions 

 of pous de mer (the fea-loufe, a fmall univalve Hiell), alfo of 

 fmall cheviettes (Hirimps or prawns), and a fpecics of crab, 

 of which the body is about a foot in breadth, and fourteen 

 or fifteen inches in length, the tail having nine or ten rings. 

 The flirimps are fometimes fo numerous, that forty have 

 been counted on a Hate of a foot fquare. 



None of the above animals refemble any known exifting 

 fpecies. But the moll remarkable circumltance in thele 

 imprefljons, particularly in the large crabs, is, that though 

 there be no fign of the body having been crulhcd, yet it 

 can fcarcely be faid to have any thicknefs whatever. They 

 rather refemble engravings than figures in relievo, the con- 

 vexity ,of thefe crabs above a thin leaf of flate not rifing 

 more than the fourth, and fometimes not more than the 

 tenth part of a line ; nor is it perceivable that the body of 

 thefe crabs penetrates into the leaves of flate. What adds 

 to the furprife, is the nearly vertical pofition in which 

 thefe impreflions are faund in the mine. 



A feries of thefe leaves of flate may be compared to a fet 

 of books placed upon llielvcs ; and the impreflions of crabs, 

 and other marine animals, to engraved plates in the volumes : 

 they do not, in faft, occupy more thicknefs. It is equally 

 difficult to conceive how the bodies of thefe animals, tiiough 

 perfeftly defined, could be reduced to a fimplc lurface, with- 

 out thicknefs. Tliefe Hates alfo prefent bv.iutiful dendriti- 

 cal pyrites, more than a foot in extent. T'le pyrites are 

 fometimes in fmall grain?, difl'eniinated, like dull, upon the 

 furface of the Hates, where may alfo be obiervcd many little 

 ilars of felenite. 



When the blocks of Hate have been drawn, from the 

 quarry, if they are left ex;"ofod to the fun, or the open air, 

 for fome days, tliey lofe what is called the iinarry-tvaUr, 

 and then become hard and untraftible, and can only be cm- 

 ployed as building Hone. Froll produces a lingular effcft 

 on thefe blocks : while frozen they may be broken with 

 more eafe than before ; but if thawed rather quickly, they 

 arc no longer divifible : yet this quality may be rellored by 

 expofing them once more to the froll ; but if the operation 

 be often repeated, it becomes impolfible to reduce them to 

 leaves. 



Some fmaller beds of flate arc worked on the northern de- 

 clivities of the Pyrenees; but we have no account of any 

 flate-quarries on the foutherii fide, or in any part of Spain : 

 yet it can fcarcely be doubted that this rock exills in fome 

 of the mountainous parts of that kingdom. 



Only one quarry of flate is faid to be opened in Italy ; 

 it is at Lavagna, in the Hate of Genoa, and furniflics 

 flate of an excellent quality, and fo impervious, that it 



fcrvc* 



