ON THE TERTIARY FORMATIONS OF SPAIN. 11 



In the basin of the Tagus are also many large rock-salt deposits. The 

 lower group of the nagelfluhe, in which the remains of the great 

 mammifers have been found, has not yet been entirely worked out ; 

 it has been however partially examined, and from it were taken the 

 remains of elephants and mastodons preserved in the rooms of the 

 Museum of Natural History. 



The limits of this great tertiary basin have not yet been made out, 

 although some few points may be given. To the north-west it rests 

 against the primitive rocks of the chain of the Guadarrama. To the 

 south it reaches a little beyond Mora and Tembleque, extending by 

 Villatobas and Villamanrique to the slopes of the mountains of Al- 

 carria. 



*' In conclusion, it may be said that the gypseous tertiary formations 

 of these three basins are characterized in the following manner : — 



1 . By the form and colour of the crystals of gypsum, which are 

 different in each of them. 



2. In the basins of the Ebro and of the Tagus there are salt depo- 

 sits, but none in that of the Douro. 



3. In none of the three basins have any fossil vegetable remains 

 been met with, as far as I am myself aware. 



4. In all these basins freshwater mollusks are found, but in that of 

 the Tagus the genera are more varied. 



5. In the basins of the Douro and of the Tagus detached isolated 

 remains of great pachyderms have been found : these^ as yet, have 

 not been found in the basin of the Ebro." 



Basin of the Guadiana. 



There is not much to say respecting the tertiary formation which 

 is traversed by the Guadiana. It is only mentioned for one remark- 

 able peculiarity it possesses, and which distinguishes it from the three 

 above mentioned. In them, as has been said, the streams of water have 

 opened large ravines and have formed great channels, which serve as 

 beds to the actual rivers ; this circumstance has exposed the whole 

 formation, thus greatly facilitating the study of the naturalist. This 

 is not the case in the basin of the Guadiana ; the surface or upper 

 stratum is there as yet almost untouched ; neither rivers nor constant 

 streams flow over it ; consequently in the rainy seasons, and during 

 the melting of the snows of the Sierra Morena, the water flows indis- 

 criminately in all directions. In order to guard against the mischief, 

 some village districts have opened great drains to carry it off, thus 

 protecting their fields and their cottages in times of excessive rains. 

 But if there are no superficial streams, internal or subterranean streams 

 are very abundant. The phsenomenon of the *'ojos de Guadiana" 

 recurs in many instances, although on a smaller scale, and wherever a 

 well is opened, there is a certainty beforehand of meeting with water 

 at a very inconsiderable depth. 



The cause of this phsenomenon appears to the author to be partly 

 attributable to the existence of certain subterranean strata of cal- 

 careous tuff in some districts forming hollows and cavities, which are 



