Geology of England and Wales. 153 
loaded with carbonate of lime, held in solution by excess of 
carbonic acid, as to deposit it as a tufa upon the adjacent 
rock, or incrust substances accidentally immersed; such are 
the petrifying springs of Matlock, Middleton, &c. 
We now reach the lowest member of the carboniferous or 
medial series of rocks, which, from its priority of deposition, 
is termed old red sandstone; it is sometimes separated from 
the limestone by a layer of shale ; it isa mechanical aggregate, 
constituted apparently of abraded quartz, mica, and felspar, 
containing fragments of quartz and slate; sometimes its tex- 
ture is slaty and fine grained ; at others it passes into a con- 
glomerate. Its colour is dark iron-red, brown, or gray, and it 
usually passes in its lower strata, by an insensible gradation, into 
the greywacke upon which it is generally observed to repose. It 
contains few organic remains, and no important minerals. 
To this outline of the rocks associated in the carboniferous 
order, our authors subjoin an excellent abstract of their pecu- 
liarities in the principal coal districts, and conclude their 
volume with an account of the Trap Roexs occurring in asso- 
ciation with the coal measures. This seems to us the only 
objectionable chapter in the book; and as they must necessa- 
rily recur again and again to this curious and important series, 
and to the phenomena which accompany them, we could wish 
that they had not broken in upon the subject in a partial and 
unsatisfactory manner. Our readers will have observed that 
the formations hitherto described, and properly enough termed 
strata, follow each other as successive deposits, in regular and 
unvarying order; but not so with the trap rocks; they make 
their occasional appearance amidst all the strata, from the chalk 
downwards; not as regular formations, but as invading masses, 
dislocating and disjointing their neighbours, converting chalk 
into marble, sandstone into chert and jasper, coal into coak, 
and shale into siliceous shist; they occasion dykes, or faults 
and elevations of the strata, and bear every mark of igneous 
origin, and of violent and sudden assumption of their present 
situation ; among the older rocks, they also play a very important 
part, and their general history tends to clear up many difficul- 
ties connected with the granitic formations. We, therefore, 
anxiously look for our authors’ second volume, in which these 
and many other intricate and anomalous, but highly interesting 
and important, geological phenomena must necessarily be dis- 
cussed at length. These circumstances, therefore, induce us 
to defer an account of the pranks and irregularities of the trap 
rocks, and of the confusion which they create among the coal 
strata, until we are called upon more formally to contemplate their 
singularities, and to endeavour to trace their origin and effects, 
with the whole mass of information which the subject requires 
before us. 
