Mineralogy and Geology. 265 
In the third volume of our ‘ Annales de Minas,’ published in 1845, I 
have given a general account of the great lacustrine formations of cen- 
tral Spain. The most important of all, and that which has been best 
examined, is what I have called the basin of the Douro (Cuenca del 
Duero), the surfacesof which is nearly forty Spanish leagues square,— 
3400 square kilom. o the west and south it rests on the crystalline 
or metamorphic formations of Galicia and of the frontiers of Portugal, 
of the Sierra d’Avila, and of the Guadarrama; to the north on the 
Devonian beds and on the chalk of the mountains of Léon; and to the 
east on the cretaceous beds. The lacustrine basin of the Ebro is also 
very considerable, but I do not know its exact limits. It is, perhaps, 
somewhat longer, following on both sides the course of the river; but 
itis not so broad. It rests almost everywhere on chalk, and partly on 
in the centre of Spain, and includes the capital. The fetid lacustrine 
limestone of Culmenar serves, as well as the granite of the Guadarrama, 
for architectural constructions, and is also used for statuary purposes, 
on account of its preserving almost indefinitely its natural whiteness. 
We also possess other small detached lacustrine deposits in the center 
of Spain, as e. g. at Libros near Teruel, at Calatayud, where fine speci- 
mens of sulphate of magnesia are collected, at Hellin and Bénamaurel, 
already quoted for the production of sulphur, at Molina d’Aragon with 
its Neritina, nov. sp., and several other localities. 
It is well known that the surface of Spain is very mountainous and 
broken up. The onl plain of any extent is that of La Mancha in New 
Castille, from Tembleque to Santa Cruz de Mudela, a distance of twenty 
a) while being upraised to a considerable elevation, have acciden- 
ta 
a 
reversed. On seein 
all the different characters of mountains which have resulted therefrom, 
tls at once evident that many instances of eruptive rocks must 
looked for. 
ae € plutonic eruptions which have pierced the gneissoid 
rocks been already pointed out in the western districts ; the eupho- 
toon diorites, and black porphyries are more frequent in the central 
the oi ain; and the trachytes are almost exclusively confined to 
_ ‘Me shores of the Mediterranean, and particularly to the southern por- 
"ECOND Serres, Vol, XI, No. 32.—March, 1851. 
. * 
