22 HENRY S. WASHINGTON 



olivine and of augite resemble those of the other type, both as to 

 their characters and their amounts, and do not call for special 

 mention. 



Microscopic characters. — The general features shown in the thin 

 section are much like those previously described, though there are 

 some well-marked differences in the most representative specimens of 

 this type. The very rarely seen feldspar phenocrysts are of a lab- 

 radorite, highly twinned, of about the composition AbiAuj, as in 

 the previous case, and similarly no constant difference of any impor- 

 tance can be detected in the characters of the phenocrysts of augite 

 and olivine, though those of the latter are more abundant than in the 

 Monte Ponente type. 



The ground-mass, on the other hand, offers more points of differ- 

 ence. Small tabular, highly euhedral crystals of labradorite are 

 abundant, but they are usually shorter and stouter than in the other 

 rocks, and the arrangement, instead of being diverse, is typically sub- 

 parallel, giving rise to well-marked flow textures, which are especially 

 clearly developed in the blocks from the tuffs. Olivine is rarely seen 

 in the ground-mass, and the anhedra of augite and magnetite are 

 closely like those described above. The abundance of dark, dusty 

 grains is seen only in one or two cases, and these transitional toward 

 the preceding type, but the interstitial, colorless base is here less apt 

 to be isotropic and often shows a faint birefringence, indicating the 

 presence of the nephelite shown in the norm. Its amount, however, 

 is never very great. 



Mode. — Owing to the impossibiHty of a satisfactory appHcation of 

 the Rosiwal method, the mode of the specimen analyzed was estimated 

 by readjustment from the norm, and is approximately as in I below, 

 that of the other type being repeated in II for comparison. 



Labradorite (AbiArii).. 



Nephelite 



Augite 



Olivine 



Magnetite (titaniferous) . 

 Apatite 



50 



5 



20 



14 



57 

 o 



26 



6 



