SLA 



SLAPER's Bay, in Geography, a bay on the W. coaft 

 of Africa; lo miles E. of Cape Blanco. 



SL ARABIA, a town of Afiatic Turkey, in Natolia; 

 28 miles N.N.W. of Efldr-Shehr. 



SLASHER, in Rural Economy, a term fometimes ap- 

 plied to a plafher of hedges. 



SLATCH, at Sea. When a rope or cable hangs flack, 

 the feamen call the middle part, which hangs down, the 

 Jlalcb of the cable, or rope : fo alfo, after long foul weather, 

 if there come a fmall interval of fair, they fay, this is a 

 Jlatch of fair weather. But the term is generally applied to 

 the period of a tranfitory breeze of wind. 



SLATE, Argillaceous Schiflus and Argillite of Kirwan ; 

 Clay-Jlale, Jamefon ; Thonfchiefer, Werner ; Schijte Argi- 

 /eu.v, Fr. ; in Mineralogy and Geology, one of the great 

 rock-formations compofing the cruft of the globe, of which 

 roof-flate is a well-known variety. The flaty or fchiftofe 

 ilruiSure is common to numerous rocks, which differ from 

 each other, both in their geological pofition and confti- 

 tuent parts ; hence we have a great variety of flate-rocks, 

 amongft rocks which have been denominated primary, 

 tranfition, and fecondary. (See Rocks.) The term 

 flate appears to have a more appropriate meaning, when 

 reftrifted to thofe rocks which, in their compofition and 

 ftrufture, are moft nearly allied to roof-flate or the clay- 

 flate of Werner. 



The moit common colours of flate are a yellowi(h-grey, a 

 greenifli-grey, or a dark-blue or purple-grey : fome varie- 

 ties incline to a red, and others to a black-grey. Slate of 

 the moft perfeft kind has a gliftening luftre. It paffes by 

 gradation into mica-flate in the rocks called primary ; and 

 in rocks confidered as fecondary, it paffes into flate-clay or 

 ftiale. 



The ftrufture of flate-rocks en maje is tabular, the fmall 

 ftr\ifture laminar, the cleavage of the lamina; being parallel 

 with the tables ; hence flate has been reprefentedby fome 

 geologifts as moil diitindly ftratified ; and the flaty cleav- 

 age is faid to be always parallel with the ftrata ; but this 

 we believe to be a millake, arifing from confounding the 

 divifions or joints of the tabular maffes with the inclination 

 of the beds. In the flate-quarries of Weftmoreland and 

 Cumberland, we have invariably obferved the tables or divi- 

 fions of the flate rife at a more elevated angle than the 

 bed of which they form a part ; and they are fometimes per- 

 pendicular, when the true dip of the bed is not more than 

 forty degrees. 



The flaty ftrufture of the ftone is the refult of a pro- 

 cefs analogous to cryltallization, by which the flate is di- 

 vided into rhomboidal blocks or tables : between the bottom 

 of one block and the top of another, there is frequently 

 a feam of clay, which forms, what the workmen call, the 

 foot of the flate. The flate will divide to this feam in a 

 diredtion nearly at right angles with it ; and fometimes be- 

 tween the upper and the lower block, there will be a thin 

 horizontal itratum of flate occupying the place of the clay. 

 This proves that the flate is not formed by mechanical 

 ftratification, and that the elevation of tlie tabular maffes 

 IS unconnefted with the elevation of the rock itfelf. 



Slate-rocks vary in hardnefs, but they yield to the knife. 

 Tiiey conlilt of an intimate intermixture, in various propor- 

 tions, of iiliceous earth, alumine, and iron ; and fometimes 

 contam a portion of lime, magnefia, manganefe, and bitu- 

 men. Slate forms entire mountains, and mountain chains, 

 and fometimes dillina beds, alternating with other rocks. 

 It moft frequently rcfts upon granite, gneifs, or mica- 

 flate. 



Slate-rocks are frequently interfeaed by veins of quartz, 



SLA 



and by metallic veins of lead, cobalt, and filver ; it alfo 

 contains beds of copper pyrites, red copper-ore, copper- 

 green, malachite, iron pyrites, magnetic pyrites, glance 

 cobalt, grey cobalt -ore, arfenical pyrites, blende and galena. 

 Tiie tin-veins in Cornwall fometimes pafs through the 

 killas, which is a variety of flate-rock. 



Various rocks frequently occur imbedded in flate, parti- 

 cularly whet-flate, or hone, chlorite-flate, talcous-flate, 

 drawing-flate, and alum-flate. Beds of flinty flate occur 

 in this rock, and alternate with as well as graduate into it. 

 Flinty flate appears to be flate in which a large propor- 

 tion of filiceous earth is combined, until it approaches 

 nearly to the nature of chert or flint. Organic remains 

 are occafionally found in flate, on which account it is not 

 confidered by many geol, gifts as a primary rock, if indeed 

 there be any to which that term is appropriate. The 

 flate which contains organic remains has been called by 

 the Germans the newer clay-Jl::'.:, and tranfttion-Jlale ; but 

 remains of vegetables have ben difcovered in the flate of 

 the Higher Alps in the vicinity of Mont Blanc, which may 

 be fuppofed to have a better claim to the rank of primary 

 than any other flate-rocks in Europe. 



In a very large proportion of flate-rocks, the bafis of the 

 flate is intermixed with particles or fragments of other 

 rocks ;. and when thefe are fufficiently large to be difcerned 

 without a lens, this kind of rock has received from the 

 Germans the name of grauwacke flate. It is generally in- 

 cumbent on the rocks denominated primary, and is fup- 

 pofed to be of more recent origin than the finer flate ; but 

 late obfervations have fliewn that grauwacke is fometimes 

 covered with granite and mica-flate. The killas of Corn- 

 wall appears, in many fituations, to approach the nature of 

 mica-flate, having a (hining luftre, and a light grey colour, 

 as if compofed of an intimate intermixture of clay-flate 

 with mica-flate. In Cornwall and Devonfliire it occupies, 

 in general, the fame relative pofition with granite, in which 

 mica-flate freqtiently occurs in other diftrifts refting im- 

 mediately upon it. The metallic veins which interfeft thefe 

 rocks are principally filled with ores of tin and copper, 

 imbedded in a matrix of quartz ; but it is obferved, that the 

 contents of the vein frequently vary as it paffes through 

 different rocks : if it contain tin-ore in the granite, it will 

 change to copper-ores in the killas ; and if tin be moft 

 abundant in the killas, the vein will change its contents as 

 it pafles into the granite. Veins of granite are alfo ob- 

 ferved to ftioot up into the killas, which is a proof that 

 this kind of flate-rock is at leaft as ancient as the granite. 

 Tliis faft has given rife to fome controverfy among geolo- 

 gifts, the difciples of Werner contending that the granite 

 in the flate or killas was not in veins, but formed ridges 

 or inequalities rifing above the furface, which had been 

 inclofed by lubfeqtient depofitions of the flate. For this 

 explanation there can be no better reafon given than its 

 convenience in fuiting a particular hypothefis refpefting 

 the formation of thefe rocks. Among other fads, which 

 might be cited to prove that the granite veins penetrate 

 the flate, there is one defcribed in the " Annals of 

 Philofophy," May 1814, which appears decifive. It occurs 

 in Toufchole, in Cornwall. " The flate or killas is of a 

 greyifti dark colour, rather hard, but breaks into large 

 fragments in the direftion of the ftrata. The granite is of 

 a fine grain, and the felfpar is of a light flefh-colour, and con- 

 tains but a fmall portion of mica. At thejunftion, numerous 

 veins of granite may be traced from the rock of granite into 

 the flate. Some of thefe veins may be obferved upwards 

 of fifty yards, till they are loft in the fea ; and in point 

 of fize, vary from a foot and a half to lefs than an inch. 



It 



