318 T. E. Bowditch, Esq. On the Geognosy of 
smallest trace of disturbance or confusion. Continuing about a 
furlong to the northward, and descending a water-course, (about 
a mile in a direct line from the beach,) I found dikes of decom- 
posing basalt intersecting the limestone, which, from their form 
and direction, I should say had evidently descended from above ; 
and, instead of filling up from below, had flowed into the gaps 
created in the limestone, by the convulsions which rent the ori- 
ginal structure of Madeira, and preceded its new form. 
The lowest bed of the sandstone above the calcareous tufa, 
(which is the lowest visible deposit at Porto Santo, and is inter- 
sected by basaltic dikes), is hard and solid, and is used as a 
building stone. I have given a particular description, which I 
need not introduce here, of its characters and gradations. The 
upper or looser sandstone, which yielded to the fingers, contained 
in its upper and outer surface, an Ampullina (or marine Ampul- 
laria), Fig. 58 ; a large Helix, resembling the H. plicata, Fig. 57, 
but differing from the plate being on the last whorl, which does 
not advance as far into the mouth ; a still larger, wholly un- 
known to me, Fig. 56 ; and two others, the one, Fig. 50, a 
Helicella of Ferussac’s groupe Margin atae ; the other, a Heli- 
cigona, of the groupe Vortices. I found no Ampullinse amongst 
the shells of the beach, (seventeen of which being new species, 
or at least not noticed by Lamarck, and unknown to me, are 
drawn and described) ; no Bulimi any where ; and the existing 
Helices (thickly strewed over the soil formed by the calcareous 
tufa, and found very sparingly on the fig-trees in the sandy 
plain,) were specifically distinct from the considerably smaller 
ones, forming entire masses of the loose sandstone, and generi- 
eally distinct from the very large species imbedded in its surface. 
The flaky sandstone frequently formed isolated ledges or hil- 
locks of a most picturesque appearance, on the southern part of 
the plain ; numerous flakes being regularly piled on each other, 
shooting upwards from the soil in angles of 45°, and seeming to 
emulate the lofty peaks of tufa behind them ; Plate VI. B. 
Embedded on these hillocks, are numerous close-grained, indu- 
rated, cuneiform, hollow masses, with smaller lateral branches, 
which I conceive to have been formed by the sand having en- 
veloped plants or fragments of wood, subsequently and entirely 
decomposed. 
