Madeira and Porto Santo. 
317 
tion was totally different from the three which were in view on the 
northern side of the river, viz. the granite at Cintra ; the tran- 
sition limestone above Guida ; and the basalt capping the hill be- 
tween the aqueduct and the city; the two latter of which are pret- 
ty fully described in my MS. work. It was a range of Calcaire 
grossiere , about 300 feet high, and extending some miles along 
the river. It was soft, but firm, frequently very sandy, some- 
times of an orange-yellow, (especially within), but generally of 
a greenish and yellowish grey. Pebbles of silex were occasion- 
ally imbedded ; and, more frequently, masses resembling clay ; 
it soiled the fingers, — effervesced moderately,-— and seemed de- 
posited in deep horizontal beds, more compact upwards. The 
shells were so thickly imbedded, that whole masses appeared to 
be exclusively composed of them. They were all marine, with 
the exception of the Balenius montanus, and comprehended 
three species of Ostrea, (O. plicatula, O. edulis and O. canalis) ; 
the Panopgea Faujasii, the Cyprina islandica, the Pecten vul- 
garis, and P. saxatilis, with three species of Venus, Fig. 30, 31, 
32 ; three of Turritelle, Fig. 23, 24, 25 ; a Cardium, Fig. 33 ; 
a Balanus, Fig. 34 ; a Nassa, Fig. 28, and a Murex, Fig. 26, 
which, possessing no specific descriptions, I could not determine,, 
and have, therefore, drawn, with one valve of a shell of consi- 
derable size, and of an orange colour ; Fig 19, which I do not 
recognise, and a smaller one, Fig. 20, which cannot be referred 
either to Tellina, Venus, or Cytherea, but which resembles all of 
them. 
At St Vicente, on the north side of Madeira, I found a simi- 
lar limestone to that which I have before described beneath the 
basalt at Lisbon. Generally speaking, however, it is of a whiter 
colour, more crystalline in its texture, contains very little im- 
bedded siliceous matter, and scarcely any compact masses ; yet 
from analogy, and from the great depth of the bed, (being near- 
ly 700 feet from its junction with the superincumbent basalt, to 
my last glimpse of it in the bed of the torrent, nearly level with 
the sea), without a single alternation, I have no doubt of its be- 
ing transition rather than primitive limestone. Its more crystal- 
line texture is probably owing to its vicinity to the basalt. The 
drift line of the junction is horizontal ; and the limestone has 
evidently been deposited regularly and frequently, without the 
