102 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



microlites. The glass where thick appears isotropic, but where thin appears 

 to be full of globulitic devitrification products, which show slight polariza- 

 tion effects between crossed nicols. 



The original presence of glass in other basalts is considered to be indi- 

 cated b)^ the occurrence of amygdaloidal cavities, with very sharply defined 

 walls marked hj accumulations of magnetite. 



The character of one basalt points strongly toward its glassy condition. 

 It is amygdaloidal, the amygdaloidal cavities being sharply defined. The 

 groundmass contains at present no indication of the existence of any orig- 

 inally crystalline elements whatever. It is now a dense mass of felty 

 chlorite and minute epidote grains. Through this mass and around the 

 amygdaloidal cavities wind lines which are somewhat differently colored 

 from the rest of the matrix, and seem to indicate the direction of flowage. 

 The amygdules are not all elongated, though some are, and these agree in 

 direction of elongation. It is really impossible to describe the groundmass 

 so as to do justice to its appearance and convince one who has not seen it 

 of its devitrified character. The general impression it makes is that of a 

 devitrified glass, and the photomicrograph (fig. S, PL XXV) gives a 

 fairly good idea of its appearance under the microscope, and will probably 

 prove more convincing than any description that might be given. Fig. A, 

 PI. XXVII, represents a polished face of the specimen in its natural size. 



Another kind of glassy basalt is represented in this district. This rock 

 resembles the one just described, but differs from it in that it was not alto- 

 gether glassy. In it one sees long, slender, much-altered feldspar microlites 

 scattered tlu-ough the matrix. These feldspars occur in needles, which 

 fringe out at the ends. They do not give the groundmass textures usually 

 found in the basalts, but occur in sheaves and imperfect spherulitic forms; 

 the rock thus approaches in texture the variolites. The base in which the 

 feldspars lie is brownish gray, and consists of recognizable chlorite, epidote, 

 some clear mineral in minute particles, probably quartz or feldspar, or both, 

 and aggregates of yellowish granules, which are apparently of a single 

 kind and are so minute as not to permit of determination. The granules show 

 very slight polarization effects under crossed nicols, and the groundmass in 

 many places where they occur in great quantity appears almost isotropic. 

 It seems highly probable that a large portion, if not all, of the gi-ound- 

 mass was originally a glass. Further evidence of the originally glassy 



