F. N. Guild — Eruptwe Rocks in Mexico. 171 



<^ — > -, Class III, Salfemane 



Fern 37-65 -^ 3 ' 5 

 F 



~ =61-74, >-, Ordero, Gallare 



vtf 1 



K + Na 2 80 5 3 n 



' CaO = 6~9' < 3" > 5~ ' g ' Com P tonase 



< — > — , Subrang 4, Comptonose 



Na,0 ~~60' ^ 5 ^ 7 



Sierra Catarina, one of the most interesting features in the 

 valley of Mexico, consists of a series of crater cones rising 

 abruptly from the level plains in the southeastern part of the 

 valley. Some are in groups of four or five, while others are 

 isolated truncated cones with an average height of 400 feet. 

 Five of them were visited by the writer during the summer of 

 1905, and all but one were found to have a well denned crater 

 on the summit. Their component material varies from pure 

 volcanic sand to compact lava, while scoriaceous forms, lapilli 

 and volcanic blocks of all sizes are abundant. A compact lava 

 is found near the base of several of them but has never flowed 

 to great distances. The predominating color is black for the 

 compact varieties of lava and black or red for the more scoria- 

 ceous modifications — porphyrinic types are entirely lacking. 



Las Calderas, perhaps the best known of these crater cones, 

 is about one mile south of the railroad station of Los Reyes. 

 It is an elongated truncated cone with a sloping summit due 

 to the erosion of the rim on the southern extremity. The 

 cone contains two craters with a narrow rim between them. 

 Barometric observations gave the following elevations : 

 Highest point on the rim, 650 feet above the plain. 

 Lowest point on the rim, 275 feet above the plain. 

 The bottom of the crater was found to be at practically the 

 same level as the surrounding plain, its diameter is about 1,500 

 feet and its interior walls are quite precipitous. 



This cone consists almost entirely of a yellowish gray strati- 

 fied tuff of about the hardness of adobe brick and with an 

 occasional volcanic block embedded in it. Microscopically the 

 ash is made up mostly of transparent glass with some micro- 

 lites of feldspar and decomposed particles of ferromagnesian 

 minerals. The volcanic block mentioned above contains small 

 and rather scattered rods of feldspar in an exceedingly dark, 

 glassy, structureless groundmass. Ferromagnesian minerals 

 are present only in isolated grains. The rock is of too vitreous 

 a nature to admit of accurate classification from its mineral con- 

 stituents. 



Directly southwest of Las Calderas is a series of crater cones 

 the best formed of which is called Cerro de Catarina. The 



