254 Dr Hiszert on the Limestone of Burdiehouse, 
Such being the plausible origin of carboniferous beds, the next 
important circumstance to notice is their alternation. And on 
this question the following considerations are suggested. 
A sandstone, of itself, may simply indicate sand, which had 
been transported by natural causes, or familiar terraqueous con- 
vulsions ;—it may indicate a quiet transportation of its ingredients 
to the beds of lakes or seas, and as quiet a deposition. Argilla- 
ceous shale, also, may be regarded as a tranquil deposit of silt in 
some lake, or sea; while limestone, whether it occurs beneath 
marine or fresh waters, may be the testimony of some calcareous 
elaboration through deep fissures incidental to the fractured state 
of our globe. And, when we find alternations of these deposits, 
it is possible that we may be presented with no phenomena which 
are not familiar to us at the present day. The river, for instance, 
which at one period: deposited sand, ‘may, from a change of its 
course, which: frequently happens, ‘be fraught with silt, or clay. 
Meteoric agencies may interfere, which would bury a deposit 
under a deep accumulation of distantly transported sand or mud. 
Lastly, new voleanos may open out fissures in the crust of the 
globe, and thus give rise to new elaborations of calcareous mat- 
ter. That some few of the alternations of sandstone, shale, and 
limestone, which we find among coal-fields are attributable to this 
kind of agency, 50 doubt: whatever can subsist. 
But the inquiry is very different, when we find certain beds 
of fluviatile origin alternate with others of marine origin ; as, for 
instance, shale-beds which contain numerous plants and the fresh- 
water unio, alternating with beds which enclose marine remains, 
such as encrinites, corallines, &c. &c. 
As far as information is communicated by alternations of this 
character, no inference can be drawn, except that the land which 
formed the basin of a fresh-water lake had, by’a terraqueous de- 
pression, become the bed of the ocean ;—or, vice versa, the land 
which formed the bed of the ocean had, by a subsequent eleva- 
tion, become the basin of an inland lake. 
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