10 GEOLOGICAL MEMOIRS. 



Basin of the Ebro. 



The Ebro also during a portion of its course traverses a tertiary 

 formation exactly resembling that of the basin of the Douro. The 

 siliceous freshwater limestone, the gypseous beds, and those of the 

 nagelfluhe, the clay and loose sand, alternate in the same order and 

 manner ; the remains of moUusks which they contam are the same, viz. 

 Lymnceus socialis and Planorbis carinata. Nevertheless the tertiary 

 formation of the Ebro has some peculiar features which distinguish 

 it from that of the Douro : in the first place, all the rocks which 

 constitute the tertiary formation of the Ebro have generally a red- 

 dish-brown tinge caused by an oxide of iron ; the gypsums also have 

 the same colour. 2. The form of the crystals of selenite is differ- 

 ent. 3. In the gypseous series, and in the lower portion, it contains 

 great saline deposits, which are not found in the basin of the Douro. 

 4. No remains of large mammifers have yet been fomid ; for although 

 a vast deposit of the remains of Equus piimigeniiis and of Bos have 

 been found at Concud in Aragon, forming breccias which fill certain 

 crevices in the tertiary soil, they must belong to a more modern 

 period. 



The author is unable to give the limits of this basin ; he merely 

 observes that the beds, which are clearly horizontal, rest on the north, 

 against the flanks of the Pyrenees. To the south it extends to Alama, 

 and on the west to the sloping sides of Moncayo. The great desert 

 of Navarre, called Las Bardenas, forms part of the tertiary district. 

 Here there must also have existed during that period a great fresh- 

 water lake, probably several of smaller extent, connected with each 

 other, as is now the case in the Alps. 



Basin of the Tagus. 



The tertiary formation of the basin of the Tagus is entirely analo- 

 gous to the two above mentioned, but more closely resembles that of 

 the Douro. The siliceous freshwater limestone, which as we have 

 observed occupies the upper portion, may be seen in the plain of To- 

 rija, in the neighbourhood of Guadalajara and Alcala, in Vacia-Ma- 

 drid, Colmenar de Oreja, Tembleque, and various other places. It 

 must be remarked, however, that the remains of mollusks hitherto 

 found in it are more varied, inasmuch as besides those already men- 

 tioned, it also contains Helices, and perhaps some other genera. The 

 range of hills of Vallecas and the soil of Vicalbaro undoubtedly be- 

 long to this upper series of freshwater limestone, as we have already 

 observed respecting the neighbourhood of Arevalo in the basin of the 

 Douro, which it exactly resembles. 



The currents of the Tagus and of its affluents have in many places 

 laid bare the gypseous group, as may be seen immediately outside the 

 gate of Atocha ; but the clays and marls of this series have a dark 

 blackish colour, as is sometimes also the case with the gypsum, which 

 crystalUzes in a distinct manner from that of the Douro or the Ebro. 



