﻿of the Republic of Salvador 1 , Central America. 2T 



In the collection are rocks showing a wide variation in min- 

 eral composition ranging from very basic to highly acidic 

 forms, from those rich in olivine to others carrying an abund- 

 ance of quartz. They can all be classified under the following 

 heads : basalt, pyroxene-andesite, hornblende-pyroxene-andsite, 

 hornblende-mica-andesite, dacite and possibly rhyolite. Basalt 

 and dacite are better represented in the collection than the other 

 types. Typical hornblende-andesite, augite-andesite and rhyo- 

 lite are not found among the specimens studied, and the horn- 

 blende-mica-andesites are closely connected with the dacite 

 by the presence of small amounts of porphyritic quartz. 



Basalt. — These rocks present quite a variety of forms, differ- 

 ing in compactness from dense to vesicular and scoriaceous, 

 from glassy to crystalline. In color they vary from black to 

 dark-gray, sometimes mottled and red. Some are crowded 

 with small porphyritic crystals of feldspar and olivine, while 

 others are nearly free from macroscopic crystals. The same 

 variableness is observed in thin sections, though the majority 

 of the specimens show a fine-grained groundmass, either glassy 

 and microlitic or microcrystalline, composed of plagioclase,. 

 augite and magnetite. Glass, when present, is either colorless 

 or brown and globulitic, occasionally the base of the ground- 

 mass is quite opaque from the abundant particles of iron oxide 

 which crowd it. The porphyritic crystals which vary in actual 

 and relative amounts are: basic plagioclase, olivine, augite, 

 magnetite and sometimes hypersthene. The plagioclase, which 

 fills many of the basalts in small porphyritic crystals, appears 

 to be of the more basic species and carries much glass and 

 groundmass in minute inclusions. These feldspars specially 

 characterize the basalt thrown out from San Miguel on Janu- 

 ary 25, 1884, the most important eruption from this volcano in 

 modern times. Since then the volcano has been comparatively 

 quiet, though every year giving signs of activity more or less 

 intense. Similar feldspars occur in the basalt erupted from the 

 crater of Izalco, January 1, 1881. In both rocks the feldspar 

 crystals average 2 mm in length. Olivine is in more or less well 

 developed individuals of small size and of the usual form. It 

 occurs in all stages of decomposition, from fresh to wholly 

 decomposed. In most of the specimens it is quite abundant, 

 but decreases gradually to those in which only an occasional 

 individual is found, hypersthene at the same time increasing in 

 amount where the rock is porphyritic, noticeably in the region 

 of Sensuntepeque. 



Augite, in imperfect crystals and grains, is much less abun- 

 dant among the porphyritic crystals than as a constituent of 

 the groundmass. It is pale, yellowish green in thin section, 

 and the larger individuals occasionally enclose olivine besides 



