212 GEOLOGY OF PART OF CUBA. 
stant tendency to encrust the walls, proportionally contracts the areas of the chambers. 
Already the entrances to several of them are thus for ever closed. Upon the floors of 
these caverns are strewed and accumulated to a thickness we had no means of estimating, 
myriads of dead snail and other shells, intermixed with the dung of thousands of bats. 
We felt assured that in this brick red mould, to which the bats have, for ages, contri- 
buted :—in the countless multitude of dead and decomposing land shells, for which this 
cave seems the tomb or charnel house; and in the slow, yet certain, conversion of the 
whole mass into a new and solid crystalline rock, we saw the immediate origin of those 
beds of shelly and sparry limestone, at first sight so perfectly inexplicable, which we now 
proceed to describe. 
LAND SHELL LIMESTONE. 
At numerous points during the ascent to the mountain of La Silla, we observed coarsely 
crystallized carbonate of lime, of a red tinge, crowded with fossil univalve shells, which 
at the first glance seemed to resemble the tertiary. We were the more readily led to 
adopt this hasty opinion, because the observations of Humboldt and De la Beche, in other 
positions within the troptes, had led us to look out for calcareous formations of no very 
ancient date. We did not immediately discover the true nature and origin of this fossi- 
liferous lime rock of La Silla. Subsequently we perceived that it there occupied what 
had once been open spaces, fissures, and even caverns, within the compact ancient lime- 
stone of this mountain. Occasionally, it was only a few inches thick. At one point it 
was from ten to thirty feet in thickness, and fifty feet in breadth; externally visible. ‘The 
latter occurred—and this is important to notice,—at the height of only fifteen feet below 
the highest summit of the precipitous ridge. It here consisted of numerous layers, most 
of which emitted an offensive animal odour, when fractured. 
After two or three other visits to La Silla, we traced with facility the extent of several 
of these shelly deposites. We acquired an insight into their origin: and we became fa- 
miliar with the process by which they were consolidated. We perceived that these 
fossilized univalves were, in fact merely contemporaneous with living genera and spe- 
cies:—that they were referrible to Jand shells, such as now every where abound in the 
mountain. and that crawled beneath our feet; to the number of at least nine or ten spe- 
cies. Their inhabitants, at certain seasons perhaps, are wont to retire into the dark 
caves and the extensive mountain fissures; and some, probably, wholly existed in those 
sequestered situations. There the dead and unoccupied shells lie in countless numbers ; 
mixed with that red soil of which we have spoken, as derived from the dung of bats. 
By the stalagmitical process, going on in the caves, in their walls and floors, the shells 
and other substances, fortuitously within its influence, become enveloped in compact 
and crystalline carbonate of lime, resembling a far more ancient rock. The operations 
of infiltration and consolidation, and the enclosing of multitudes of shells, is seen uni- 
formly proceeding, until fissures, and caverns, and irregularities, are alike filled up, and 
until, finally, the newer limestone, overflows the surface of the older. 
We were, at times, when examining this, very recent, formation, reminded of the fetid 
