PETROGRAPHICAL NOTES. 9 



ill the cretaceous sandstones and conglomerates of Punta 

 Banda, which would indicate a mesozoic or )3re-mesozoic 

 age of these eruptions. In view of the fact, however, that 

 the flow structure is still clearly discernible and well pre- 

 served, and that tufas are of frequent occurrence, we are 

 justified in regarding the age of the flows as mesozoic, even 

 perhaps, late mesozoic. The center from which these flows 

 originated was probably situated twenty or twentj^-five miles 

 inland; at any rate the porphyrites do not continue in an 

 easterly direction any further than this. 



Yarious structural varieties may be noted among these 

 rocks, as well as difl'erences in the macroscopical appear- 

 ance. As a rule the quartz-porphyrites have a very hard, 

 dense and flinty groundmass of black or brown color, in 

 which lie embedded quartz grains and small tabular feld- 

 spar crystals. 



Detailed Description. 



1. The rocks from Punta Ensenada, the blufl* just north- 

 west of the town of that name, correspond in macroscopical 

 appearance well with the description just given. The 

 groundmass is chocolate-brown, and contains many small, 

 white feldspar crystals, not exceeding 0.5 mm. in length. 

 No porphyritic quartz. Under the microscope the feldspars 

 appear in prismatic sections, all of which show polysyn- 

 thetic twin-structure. Isolated fragments gave with hydro- 

 silico-fluoric acid abundant Na reaction, and but very little 

 Ca; according to this the feldspar w^ould probably be an oligo- 

 clase. No bisilicate or porphyritic quartz. The ground- 

 mass is holocrystalline, composed of a fine aggregate of 

 quartz and feldspar; it often has a microfluidal appearance 

 by the arrangement of the opacite in streaks and bands. 



Sometimes there are curved streaks of a clearer mass, 

 which, between crossed nicols, shows perpendicular, double- 

 refracting fibres, evidently consisting of feldspar. The feld- 

 spar grains of the groundmass are not striated. In spite of 



