SLATE. 



large maiies, and fplit with proper tools by tlie workmen. 

 Though the fpecific gravity of Weftmorcland flate from 

 different quarries is nearly the fame, yet ull the forts 

 are not capable of being fplit into an equal degree of 

 thinnefs. Here alfo the quality varies with the depth 

 uf the quarry, that being the bell which is raifed from the 

 greatell depth. 



The grey-blue (late from Donvball, in Cornwall, weighs 

 only 2JI2 ouncej to the cubic foot, which is confiderably 

 lighter than that of Wcltmoreland. This flate is generally 

 preferred to any other for its lightnefs, and enduring the 

 weather ; but Dr. Watfon is of opinion, that in durability 

 It does not excel that of Weftmoreland. The Donyball 

 (late IS fplit into laminx about one-eighth of an inch thick 

 when it is applied to the covering of a roof; it then weigh.-, 

 rather more than 26 ounces to the fquare foot. The pale 

 blue flate from Amblcfide, in Weftmoreland, weighs about 

 two oup.ccs more in the fquare foot than the former. In 

 many inftances, we believe flate of a thinner kind is ufed in 

 ieveral modern buildings to fave the expence of timber in 

 •.he roof, where cheapnefs rather than durability is a princi- 

 ;)al objeft with the architedl. According to an ettimate of 

 Dr. Watfon, the relative weights of a covering of the fol- 

 lowing different materials, for forty-two fquare yards of 

 roof, are as under : 



Copper 

 Fine flate 

 Lead 



Coarfe flate 

 Tile 



Cwt. 



4 

 26 

 27 

 36 

 54 



A ton of fine flate will cover a larger furface than a ton 

 oi lead ; and where there is water-carriage, does not colt one- 

 fourth of the price. Slate might, therefore, be ufed gene- 

 rally indead of lead, with great advantage. Watfon's 

 Chemical Edays, vol. iv. 



The moll extenfive flate-quarries in Great Britain are 

 the property of lord Penryn, near Bangor, in Caernarvon- 

 fhire. There is a rail-road formed from the quarries to the 

 fea for the conveyance of the flate, which is of an excel- 

 lent quality, and is fent to various part^ of the world. The 

 motl remarkable fituation wheri; flate is procured in Cum- 

 berland, or perhaps in Great Britain, is Hourlton cragg, a 

 lofty mountain near the lake of Buttermere, about 2000 

 feet above the level of the lake, and nearly perpendicular. 

 On account of the difiiciilty of acccfs, the workmen take 

 their provifions for the week, and fleep in temporary huts 

 on the fummit. During the winter months they are gene- 

 rally involved in cloud's, and not unfrequently blocked up 

 by the (now. The flate is conveyed down a zigzag path 

 cut ill the rock on fledges, one man attending to prevent 

 the acceleration of the defcent. When the flate is emptied 

 at the bottom, the fledge is carried back on his flioulders to 

 the fummit. 



There are confiderable flate-quarries near Ulverftone, in 

 Lancafhire. A coarfe flate is got near Ingleton, in York- 

 fliire, and alfo in the vicinity of Settle in that county. The 

 Ingleton flate frequently contains cubical pyrites, and is 

 iometimes covered with dcndritical pyrites. 



Alum-flate, ampcUhe of Haiiy, 13 fometimes imbedded in 

 clay-flate, but more frequently in ftratified fccondary rocks, 

 and is not effentially different from fome of the coal fliales. 

 (See Ali'M.) The alum-flatc, or alum-fhale of Whitby, 

 is of vail and unknown thickncfs, forming the bafe of the 

 Cleveland Hills in the North Riding of Yorkfhire, cxtend- 

 iiiir about thirty miles call and well from Robinhood'a bay, 



Vy... xxxiii. 



to Ginfljorough, Stokefly, and Ofinotherly. On die fouth 

 fide of theClevcland Hills the alum-flate is principally covered 

 by fand-ftone and marie. On the north-eaft, the alum-rock 

 extends along the coall, about thirty miles from Robiiihood's 

 bay to Huntcliff. The height of the alum-cliffs, which are 

 perpendicular from the fea, varies from 100 to 140 yards. 

 Whitby abbey Hands near an awful precipice of alum-flate or 

 rock, which h undermined by every returning tide. At 

 low water the alum-rock may be feen extending far to the 

 ealt, forming a flat pavement, on which the obferver may 

 walk fecure, treading at almoll every flep on the organic re.. 

 mains of the inhabitants of a former world, which are 

 abundantly difl'eminated through the whole mafs beneath, 

 and projeding from the fides of the black and frowninjr 

 cliffs above. The alum-flate has been perforated near the 

 fea to the depth of 1 30 yards, without difcovering the fub- 

 jacent rock, to which we may add the height of the cliff* 

 above, which will make a total thicknefs exceeding 220 

 yards. The upper parts of the bed are found more pro- 

 dudlive of alum than the lower. From the quantity of 

 pyrites contained in tliis rock, it fom.etimes takes tire fpon- 

 taneoufly, when a heap of it which has fallen from the Cliffi 

 becomes moifleiied with fea-water. 



The animal remains are fcattercd through every part of 

 the rock. They confill principally of numerous ammonites, 

 nautilites, bclemnites, foffile vertebra (fuppofed to belong to 

 the fliark), with bivalve (hells, foffile wood, and jet. Mr. 

 Eakewell confiders the alum-fhale or flate, and the ftrata 

 which cover it, as a peculiar local formation fubjacent to 

 the roe-flone, but above the coal-formation of Yorkfliire 

 or Durham. The alum-flatc is in faft a thin bed of in- 

 durated pyritous flate-clay, differing little, except in geo- 

 logical pofition and its organic remains, from fome of the 

 coal-(hales. 



The alum-flate of Wliitby has a very dark grey colour, a 

 flaty flrudlure, and rather a filky luflre ; it fplits, by ex- 

 pofure to the atmofphcre, into very thin lamini ; it varies in 

 hardnefs, but is all fofter than roof-flate. The particular 

 advantage which the country near Whitby poffefles for the 

 manufadlure of alum, h derived from the alum-flate riting 

 in precipitous cliff?, which afford facilities for working and 

 burning the ftone. Though many of the coal-fhales might 

 yield an equal quantity of alum, the difficulty of raifing 

 them to the furface would in moll fituations be too great to 

 repay the expence. The alum-flate is piled in vaft heaps 

 and fet fire to ; a flow combuiUon is continued for fevcral 

 months, by the inflammable matter combined with the ftone. 

 The faline contents are extradted by folution, a Imall quan- 

 tity of potalh is added, and the alum is cryllallized by eva- 

 poration. ( Bake well's Introduftion to Geology.) From 

 the alum-rock of Yorkfliire, nearly all the alum of com- 

 merce in England is produced. 



According to the analylis of Klaproth, alum-flate 

 contains 



Sulphur 



Carbon 



Alumine 



Silex - 



Black oxyd of iron 



0.2S 

 1.96 

 1.60 

 4.00 

 64 



Sulphate of iron, lime, and potafh, each 15 

 Water 70 



M. Klaproth remark.^, that the fulphur in the alum-flate 

 which he analyfcd was not united to the iron but to the car- 

 bon, in a manner at prcfent unknown. In the ahmi-flate of 

 Whitby, we bilieve the fulphur is combined ijoth with the 

 iron and carbon. 



P Drawing- 



