VARIED ASPECTS OF SLATE, 39 
lousely informed on the matter is termed clay-slate, 
and which I have above recognised in a general way 
as being fossiliferous, must receive this appellation. 
From what I have just stated, it might be gathered 
that the interruptions to the courses of the different 
varieties Of slate are not sudden, but rather the re- 
verse. ‘This circumstance indeed, is remarkable 
amongst the features of this rock considered as a 
whole. Mica-slate, clay-slate, (used in a limited 
sense) roofing-slate, grey-dunstone and greywacke 
slate, are observed to pass gradually into one another, 
and to reciprocate each other’s qualities. Again, 
greywackeé-slate, flinty-slate and greywacké gradu- 
ate into one another. Roofing-slate is found spa- 
ringly, and in small patches. It obviously passes 
into the general mass of clay-slate, of which, not- 
withstanding its containing organic remains, | have 
above surmised that it may in propriety be ranked 
apart. ‘The general body of clay-slate assumes a 
great variety of aspects, which are manifestly gradu- 
ated, sometimes approximating the typical roofing 
slate and sometimes degenerating into a loose, brown 
rubble, or becoming indurated or closely impacted 
constituting “grey dunstone;” notunfrequently also 
running into decided greywacké, either in small 
patches or even in extensive beds. Altogether, 
these last named rocks form the generality of our 
hills, which are round-backed. In situations where 
the slate is in tolerable sized fragments, fossils are 
found in plenty. Although I have mentioned certain 
kinds of slate as being fossiliferous, I believe that 
no fixed rule can be laid down on this subject, for 
the same kind of slate will in one place exhibit these 
remains, and*in another be destitute of them. 
Greywacké-slate is most constant in this respect. 
The rubbly and loose kinds of clay-slate, and the 
best kinds of roofing-slate, but seldom contain them. 
The generality of our slate, that namely, to which I 
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