294 REACTION OF THE INTERIOR OP THE EARTH 



which I have, drawn. Each has retained its former 

 name, Kio de San Pedro and Eio de Cuitimba. I found 

 the temperature of the steaming waters at this point 

 126'9. The waters, during their long course, have 

 become warmed only, but not acidulated. The test- 

 papers, which I always carried about with me, were not 

 at all altered by them ; but farther on, near the 

 Hacienda de la Presentacion, towards the Sierra de las 

 Canoas, there gushes forth a spring impregnated with 

 sulphuretted hydrogen, forming a basin twenty feet 

 broad. 



In order to obtain a clear idea of the complicated 

 form in relief of the space of ground in which such re- 

 markable upheavals took place, we must distinguish, 

 hypsometrically and morphologically, (1) the situation 

 of the volcanic system of Jorullo in relation to the mean 

 level of the high plain of Mexico, (2) the convexity of 

 the Malpais, which is covered by thousands of hornitos, 

 (3) the fissure, upon which six volcanic mountain-masses 

 have risen. 



On the western declivity of the Cordillera Cen- 

 tral de Mexico, which runs from S.S.E. to N.N.W., 

 the plain of the Playas de Jorullo (only about 2560 

 feet above the level of the Pacific) forms one of the 

 horizontal mountain-stages, or terraces, which every- 

 where in the Cordilleras interrupt the descent, and 

 therefore retard more or less the change of temperature 

 experienced. When from the central plateau of Mexico, 

 7000 French feet (7460 English) in mean height, we 

 descend to the wheat-fields of Valladolid de Michuacan, 

 to the pretty Lake of Patzcuaro with its small inhabited 

 island Janicho, and into the meadows around Santiago 



