94 



on the rock of porphyry called el Pefwl de los 

 Bannos, on the bank of the lake of Mexico. 



Two kinds of sand cover the shore ; one is 

 black and basaltic, the other white and quart- 

 zose. In a place exposed to the rays of the sun, 

 the first raised the thermometer to 51* 2° (41° R.) 

 and the second to 40° (32° R.) The temperature 

 of the air in the shade was 27' 7° or 7*5° higher 

 than that of the air over the sea. The quartzose 

 sand contains fragments of feldspath. It is thrown 

 back by the water, and forms, in some sort, on 

 the surface of the rocks, small islets where the 

 seaweed vegetates. Fragments of granite have 

 been observed at Teneriffe; the island of Gomora, 

 from the details furnished me by Mr. Brousson- 

 net, contains a nucleus of micaceous schist * ; 

 the quartz disseminated in the sand, which we 

 found on the shore of Graciosa, is a different 

 substance from the lavas, and the trappean por- 

 phyries which are so intimately connected with 

 the volcanic productions. From these facts it 

 seems evident, that in the Canary Islands, as 

 well as on the Andes of Quito, in Auvergne, 

 Greece, and the greater part of the globe, the 

 subterraneous fires have pierced through the 

 rocks of primitive formation. In treating here- 

 after of the great number of warm springs, which 



* Glimmerschiefer, 



