of the Cuzco Valley, Peru. 



97 



these dikes, or similar ones in the immediate neighborhood 

 mark the source of the extrusive masses. 



In the hand specimen flow structures and zones consisting 

 chiefly of lithophysse are conspicuous features of the rock from 

 Sierra-Bella. Under the microscope the extrusive variety is 



• Fig. 42. 



Fig. 42. View looking down the Urubamba River at Sierra-Bella, 

 flow on the right. 



Lava 



seen to consist of labradorite in lath-shaped crystal fragments, 

 biotite in narrow elongated shreds of dark-brown color, and 

 widely scattered grains of iron ore. Calcite and tridymite 

 occur as amygdules. In texture the rock is hyalopilitic — a 

 mesh composed essentially of minute laths of feldspar in a 

 groundmass of glass. In a microscopic slide prepared from a 



Am. Jour. Sci.— Fourth Series, Vol. XLT, No. 241. 



7 



-January, 1916. 



