1JTC1INATION OF STRATA. 83 



in the south of the ravine of Piedras Azules, arc separated 

 from the griinsteins and serpentine rocks that cover the 

 declivity of the mountains north of the ravine. In the 

 present state of knowledge I find difficulties almost equally 

 great in adopting either of these suppositions ; but I have 

 no doubt that, when the real grunsteins (not the hornblende- 

 griinsteins) contained in the gneiss and mica-slates, shall 

 have been more attentively examined in other places ; when 

 the basalts (with pyroxene) forming strata in primitive 

 rocks* and the diabases and amygdaloids in the transition 

 mountains, shall have been carefully studied; when the 

 texture of the masses shall have been subjected to a kind 

 of mechanical analysis, and the hornblendes better distin- 

 guished from the pyroxenes,t and the grunsteins from the 

 dolerites ; a great number of phenomena which now appear 

 isolated and obscure, will be ranged under general laws. 

 The phonolite and other rocks of igneous origin at Parapara 

 are so much the more interesting, as they indicate ancient 

 eruptions in a granite zone ; as they belong to the shore of 

 the basin of the steppes, as the basalts of Harutsh belong 

 to the shore of the desert of Sahara; and lastly, as they 

 are the only rocks of the kind we observed in the mountains 

 of the Capitania- General of Caracas, which are also destitute 

 of trachytes or trap-porphyry, basalts, and volcanic produc- 

 tions. J 



The southern declivity of the western chain is tolerably 

 steep ; the steppes, according to my barometrical measure- 

 ments, being a thousand feet lower than the bottom of the 

 basin of Aragua. From the extensive table-land of the 

 Villa de Cura we descended towards the banks of the Rio 

 Tucutunemo, which has hollowed for itself, in a serpentine 

 rock, a longitudinal valley running from east to west, at 

 nearly the same level as La Victoria. A transverse valley, 

 lying generally north and south, led us into the Llanos, by 



* For instance, at Krobsdorf, in Silesia, a stratum of basalt has been 

 recognized in the mica-Mate by two celebrated geologists, MM. von Buch 

 and Raumer. (Vom Granite des Riesengebirges, 1813.) 



t The griinsteins or diabases of the Fichtelgebirge, in Franconia, which 

 belong to the transition -slate, sometimes conain pyroxenes. 



J From the Rio Negro to the coasts of Cumana and Caracas, to tin 

 esut of the mountains of Mericia, which we did not visit. 



o 2 



