F. N. Guild — Eruptive Rocks in Mexico. 173 



approach to them from Los Keyes is over an exceedingly 

 rough bow of compact lava. 



Several specimens were collected from this elevation, one of 

 a lustrous black rock with conchoidal fracture and free from 

 cavities or inclusions. Others collected from the loose material 

 around the crater were more scoriaceous and contained many 

 cavities. Under the microscope they all show about the same 

 characteristics, the chief variation being in the relative amount 



Fig. 6. View taken from the interior of the crater of Las Calderas, show- 

 ing the walls and stratified condition of the volcanic ash. 



of glass present. The description of the block found on Las 

 Calderas applies to most of the fragments found near the sum- 

 mit of Catarina. The rock of the rough lava flow mentioned 

 above, while probably of the same mineralogical composition, 

 has quite a different texture. It is basaltic, consisting of dark 

 glass swarming with minute rods of feldspar. The rods are 

 not pressed together as is usual in these types of rock but each 

 individual appears quite distinct from its neighbor. 



Cerro Xaltepetl, located still further south, is similar to the 

 last described cone except that it is of less altitude, contains a 

 greater quantity of the liner products of volcanic activity and 

 the outflow of lava at the base is lacking, and there are strati- 

 lied beds of ash in its place. Sections prepared from volcanic 

 blocks, lapilli and scoriaceous material reveal nothing new as 

 compared with that from the other cones of the same group. 

 The crater is very shallow and probably not more than 500 



