270 FINLAY 



textured porphyrltlc habit which characterizes such intrusive 

 masses. Denser textures show but rarely along the borders 

 of the laccolith. The prevailing colors are light yellow and 

 blue gray. Portions of the rock are locally richer in silica than 

 the main mass, and show free quartz. All possible gradations 

 exist between the normal andesite and the more siliceous dacite. 

 Four types of these rocks will be described, as follows : 



Andesite, San Narciso type ; Vegonia type. 



Dacite, Florencia type ; Imogenia type. 



I. Andesite 



(a) Sail Narciso Type. — This variety of the andesite is largely 

 developed to the north and west of San Jose. 



In the field this rock presents an even gray ground-mass. 

 It is leucocratic, with the darker crystals as scattered grains but 

 sparingly developed, although basic segregations are more than 

 usually common. 



The feldspar, which is by far the most abundant mineral 

 present, is recognized in the holocrystalline ground-mass by its 

 bright cleavage faces. These show occasional twinning striations. 



Under the microscope the structure is seen to be holocrys- 

 talline porphyritic, and plagioclase, orthoclase, augite, quartz, 

 biotite, magnetite, titanite and zircon are determined as the con- 

 stituent minerals. 



The sections of plagioclase are lath-shaped pieces of labra- 

 dorite, 1.2 mm. long by .5 mm. in breadth, appearing as por- 

 phyritic crystals in a very coarsely crystalline ground-mass 

 which consists of orthoclase and quartz. (See PI. XII, fig. i.) 

 The labradorite Ab^An^ appears in fresh crystals, largely free 

 from the effects of weathering. These are twinned in fine 

 lamellae on the albite law, with many pericline lamellae. When 

 viewed between crossed nicols in the 45° position, one half of 

 the crystal usually appears blue gray, the other yellow, indi- 

 cating twinning on the carlsbad law. Zonal structure is very 

 common. The outermost rims of the plagioclase crystals are 

 often deeply changed by weathering. This material has the ap- 

 pearance of kaolinized orthoclase. 



