84 



SLATE. 



Rocks covering Transition Rocks (unconformably). 



4. Porphyry, passing into trap or green-stone. 



5. Clink-stone, passing into basalt. 



6. Basalt. 



Strata covering Transition Rocks (conformably). 



7. The coal measures.* 



Slate — of which roof-slate is a well known variety — is called by 

 the Germans Thon-scheifer or clay-slate ; by ancient English geolo- 

 gists, argillaceous schistus ; by the modern French, Phyllade. The 

 term slate is perhaps the most proper that can be used to designate 

 this rock : as the best variety of it, Roof-slate, is well known. Clay- 

 slate is a name given from an erroneous opinion respecting its con- 

 stituent parts ; and the term is liable to create much confusion, as 

 the softer kind of slate in the coal strata is called slate-clay. I shall, 

 therefore, throughout the present volume substitute the term slate 

 for clay-slate, and for slate-clay the more intelligible English term 

 shale. 



Slate rocks abound in most alpine districts, resting either on gran- 

 ite, gneiss, or mica slate. That slate which lies nearest the primary 

 rocks has a more shining lustre than the other, and partakes more 

 of the crystalline quality of mica-slate. As this rock recedes from 

 the primary, its texture is generally more earthy. Its colours are 

 various shades of gray, inclining to blue, green, purple, and red. 

 Some kinds of slate split into thin laminae, which are well known as 

 forming roof-slates. Slate rocks are commonly divided into beds of 

 various degrees of thickness, which generally are much elevated, 

 and from the natural divisions of the rock, they often form peaked 

 and serrated mountains. 



Slate has been described by former geologists as distinctly strati- 

 fied, because it splits easily into thin laminae, and the direction of the 

 lamina? is asserted to be in the direction of the beds ; but, in oppo- 

 sition to the authority of many eminent geologists, I maintain that 

 slate, unless it be of a soft or coarse kind, approaching to shale or 

 greywacke, invariably, splits in a transverse direction to that of the 

 beds, making with that direction an angle of about sixty degrees ; — 

 it has frequently two distinct cleavages. 



Few persons, perhaps, have examined more slate rocks, or con- 

 sulted more workers in slate quarries than I have; and the fact re- 

 specting its cleavage is invariably what is here stated, except in very 

 coarse greywacke-slate, and soft slate or shale. 



* The regular coal strata or coal measures, where they occur in England, sepa- 

 rate the transition from the secondary rocks. If they are classed with either, it 

 ishould be with the former. 



