164 Prof. H. Karsten on the Geological Age 



Boussingaultii is plentiful. Upon the neighbouring south- 

 east foot of the Plutonic rocks of the Sierra Nevada are de- 

 posited correspondingly stratified but unfossiliferous cal- 

 careous and aluminous rocks, in part crystalline and schistose. 



In the valleys of this Cretaceous formation Tertiary strata 

 are deposited up to at the outside 200 metres. These appear 

 to be entirely wanting on the northern slope, where, however, 

 according to the statements of the natives, limestone contain- 

 ing Belemnites occurs. 



In opposition to Acosta, Hettner (Petermann's Geogra- 

 phische Mittheil. 1885, p. 92) concludes from the direction of 

 the centre of the mountains from S.S.W. to N.N.E., that the 

 mass is a continuation of the Central Cordillera, which is like- 

 wise furnished with a nucleus of Plutonic rock, and to which 

 this line of strike under 8° N. lat. leads eastwards from the 

 river Nechi ; an opinion which is in harmony with the deve- 

 lopment of these Colombian mountain-chains as deduced by 

 me from the nature of the Neptunian strata, although the 

 contemporaneity of the two existing mountain-systems is not 

 thereby proved. 



In my Yiennese address on New Granada (Amtlicher 

 Berieht, 1856), as well as in a more recent memoir on the 

 Geology of the three northern Republics of South America*, 

 in wdiich I have brought together all that was observed by me 

 in those countries, or that has been published by other travel- 

 lers, I concluded that the mountains of Guiana are the central 

 point of the different mountain-systems of Colombia, by 

 which the direction of these mountain-chains was prescribed, 

 having been elevated as the northern (Venezuela) and western 

 (New Granada and Ecuador) borders of a great semicircular 

 fissure which was formed in the solid crust of the earth around 

 this primitive centre of upheaval, which, although it was not 

 then recognizable in its whole extent by prominent mountain- 

 masses, nevertheless traced out the direction of contempo- 

 raneous and subsequent eruptions. 



The elevatory force which gave origin to this fissure in the 

 periphery of the central mountain-mass already in existence 

 appears to have acted from the east to the west and south, 

 and indeed on the greatest scale in the north, gradually 

 becoming weaker towards the south. On the contrary, the 

 last considerable upheaval, that of the Tertiary epoch, followed 

 the opposite direction ; its greatest force was exerted in the 

 south. 



In the north the Plutonic mountain-chains which bound the 



* Geologic, de Vancienne Colombie Bolivarienne, Venezuela, Nouvelle 

 Grenade et Ecuador. — Berlin, Friedlander, 1886. 



