COMPACT GRAUWACKE. 
46 
GRAUWACKE ROCK. 
The true greywacke rock, or compact greywacke, 
(“ blue dun” of quarrymen) from which our fossili- 
ferous series of rocks has in later times been named, 
is found in intimate connexion with sandstones, 
slates, and limestone. It consists in almost every 
case, of inconsiderable accumulations of, for the 
most part, dense material, used for building and re¬ 
pair of roads, is of a light blue colour, and very fine 
grained ; its hills are usually low, and with one or 
two exceptions, its tracts small in aerea. It passes 
freely into slates and sandstones, and the interme¬ 
diate kinds of rock are of difficult determination ; 
saving however these cases, greywacke may be dis¬ 
tinguished from sandstone by its melting before the 
blowpipe. So manifestly is it a component of our 
fossiliferous series, that in one spot I have found it 
reposing in a lap of limestone, and connected on 
one hand to slate, and on the other to a fine grained 
sandstone ; both of which strata are themselves also 
closely joined to the lime, the former indeed being 
like the grauwacke rock a superficial bed, and rest¬ 
ing on and running into the limestone. Grauwacke 
is also found to run in veins through slate, and 
graduating with it at the same time. It is thought 
probable, that its highly comminuted texture obvi¬ 
ated the display of the same animal remains which 
its collateral strata are often found to envelope, and 
yet, this rule is hardly worthy of regard, for, the 
slate rocks themselves, considered by some as formed 
of a highly comminuted detritus of other strata, 
includes fossils in a perfect state, and the sandstone 
of South Devon, obviously constituted of aggregated 
particles, incloses perfect specimens of alcyonia, 
turbinoliae, corals, &c. which in all probability were 
in existence, at the time the turbid mass consolida¬ 
ted around them. 
