Remews — Patt'isoiis Slate and Slate-quarries. 33 



the Slate rocks ; Slate onterpvise and trade, manufacture and Kale ; 

 each of which is treated in a concise and readable manner, and con- 

 tains a vast amount of interesting and practical information, which 

 we are not aware has previously been condensed into so small a 

 compass. 



Slate consists principally of silica, alumina, and oxide of iron, with, 

 small quantities of other substances, as lime, magnesia, alkalies, etc., 

 etc. ; is about 2^ times heavier than water, and was originally a marine 

 deposit of pure clay and sand derived from the decomposition of tho 

 older felspar-bearing rocks, but subsequently altered by mechanical 

 movements and other agencies, so as to give rise to a slaty cleavage. 

 The origin of this structure has long and frequently engaged the 

 attention of geologists and physicists ; but Mr. Pattison justly re- 

 marks that the Cambridge philosopher (Sedgwick) was the first to 

 define and accurately describe the phenomena, so as to bring the 

 facts into scientific order for practical use. 



Cleavage must not be confounded with stratification, as was done 

 by Saussure and the older observers ; for although sometimes coin- 

 ciding with it, the true lines of bedding may be frequently observed. 

 Generally speaking, any slate quarry presents three different kinds 

 of lines, — one set, usually vertical, termed joints ; another parallel to 

 each other or planes of bedding ; and a third rarely coincident with 

 either of the other two or the planes of cleavage. The mechanical 

 theory, more or less modified, has been adopted by Sharpe, Tyndall, 

 Eamsay, Sorby, and Poulett Scrope. The latter author attributes 

 slaty cleavage to extreme pressure, occasioning internal movements 

 and mutual friction in the particles of the solid mass, under which 

 they became re-arranged so as to produce lamination ; and Mr. Sorby 

 observes that fine or imperfect cleavage depends on the original 

 materials being fine or coarse. 



Slate-bearing rocks in this country are confined to the Palaeozoic 

 series, and chiefly to the strata below the Coal-measures, although 

 in the Alps slates are worked in the Lower Tertiaries. 



Slaty cleavage is found in the Cambrian rocks, in the Lower 

 Silurian, in which strata in Wales the proportion of cleaved rock to 

 the general mass appears to attain its maximum ; in the Upper 

 Silurian, as at Llangollen ; in the Devonian, as at Tintagel and Dela- 

 bole ; in the Lower Carboniferous, as in the south of Ireland, near 

 Cornwall, around Dartmoor, and near Launceston. The so-called 

 Stonesfield and Collyweston slates belong to the Oolitic rocks, and 

 are only thin laminated calcareous flagstones, which split after 

 exposure along the lines of original deposition. Cleavage, in Britain 

 at least, is less and less displayed as we ascend in the PaliBOzoic 

 rocks, and is of several epochs, for Prof. Eamsay has shewn that the 

 strongest cleavage is found in the Cambrian, Lingula, and Tremadoc 

 slates, before tho deposition of the Upper Silurian, and then a second 

 and more partial cleavage, well marked around Llangollen. 



Mr. Pattison succinctly treats of the fossils of the slate rocks, 

 showing that while the rocks themselves were originally sea-bottoms 

 and beaches, formed under similar conditions to those at present, 



VOL. VII. — NO. LXVII. 3 



