MADEIRA, 555 
ably higher. Over the surface below this line, there are small 
patches of limestone layers, consisting of calcareous sand, and great 
numbers of cylindrical stems of lime are lying around, or standing up- 
right. ‘These stems are often branching, and vary from a fraction of 
an inch to ten inches or more in diameter; yet the “ petrified forest,” 
as it is called by Mr. Bowditch, is hardly knee high in any part. The 
exterior of the concretions is arenaceous, and within, they have gene- 
rally a sandy look, with no trace of any regular structure. Some of 
the larger specimens are tubular, from the removal of the centre, 
which is less firmly compacted than the exterior. ‘They appear to be 
concretions around the root-fibres of some plants, or about the perfora- 
tions of some seashore animal. It cannot be doubted that the whole 
region was once at a lower level, and, at the time, calcareous sands, 
from triturated shells, and perhaps, also, crabs, may have been 
washed in by the sea, as beach deposits, to a certain limit above high 
water mark. It is possible that the upper ledge, on each side, was the 
water limit; but from the character of the sands and the layers of cal- 
careous sandstone, and their resemblance to the beach sand-rock on 
coral islands, the former supposition is, perhaps, more probable. 
There are numerous land-shells in the calcareous sand-rock, besides 
some of marine origin. Several of these shells are described and 
figured, in the work on Madeira, by Bowditch. Other collections 
were made by James Smith, Esq. ;* and from the examinations of Mr. 
Lowe, all but a sixth are recent species. 
The St. Vincent limestone was discovered by Bowditch, and set 
down by him as of transition age. In our brief time on the island we 
did not reach it. It occurs, according to Mr. Smith, twenty-five hun- 
dred feet above the sea, and contains corals and marine shells, which 
indicate a tertiary or post-tertiary origin. 
Large quantities of a coral limestone were seen at Funchal, which 
had been brought from small islets near Porto Santo. It so resem- 
bles, in its white colour, compactness and homogeneous texture, 
much of the coral rock of the Pacific, that the two could not be dis- 
tinguished. Some of the blocks abounded in fossilized corals, besides 
casts of shells resembling, as nearly as can be determined, recent 
species. It also contains occasional pebbles of the basaltic rocks of 
the region. 
* Proc, Geol. Soc, of London, No. 73, Dec. 6, 1840. 
