12(5 THE CRYSTAL FALLS IRON-BE AK^G DISTRICT. 



consideration, was (quartz, the tendency naturally being to replace more 

 alteraljle with less alterable minerals. 



Flattening of amygdaloidal cavities. 111 SOmB of tllC aiUygxlaloids (fig. B, PI. 



XXV) the cavities retain their circular shape, as though the rock had not 

 flowed to anv great extent. More commonly the ca^•ities are di-awii out 

 into irregular (fig. A, PI. XXV) or lenticular shapes, the long axes agreeing 

 with the direction of flowage in case their deformation resulted from this, or 

 with the direction of schistOsity in those cases where the rocks lla^•e been 

 extensively mashed. In some cases the cavities have been so extremeh' 

 flattened that the amygdules appear almost the shape of a melon seed, 

 showing a mere streak of chlorite in the sections cut perpendicular to the 

 schistosity, and in the planes of schistosity large lustrous oval areas. 



In some few of the basalts the groundmass immediately surrounding 

 the amygdules is characterized by an accumulation of feiTUginous matter. 

 Ill most cases, however, this part of the groundmass does not differ in any 

 respect from the rest of the groundmass of the basalts and points to a very 

 gradual cooling. 



ALTEKATIOX 01'" THK BASALT.S. 



The descriptions given are of the freshest and most characteristic 

 basalts. As already explained, the mineral constituents in even these 

 freshest ones have undergone a very far-reaching alteration. The rocks 

 which show a more advanced stage of alteration exhibit merely a difference 

 in degree rather than in kind, and the minerals which result are in all cases 

 the same. They are uralite, actinolite, epidote-zoisite, chlorite, white and 

 brown mica, calcite, sphene, quartz, and feldspar. 



The amount of these secondary minerals varies greatly, showing that 

 the alteration products resulting fi-om the same kind of original rock may 

 differ very materially according- to the process of metamorj)hism. 



In a general way the alteration of the basalts, as observed under the 

 microscope, has taken the following- course: Even in the rocks nearest their 

 original condition the augite has largel}' changed to uralite. The A'itreous 

 base, if any was present, has become devitrified. Rocks in this stage of 

 change still show the more imjjortant external characters of igneous rocks, 

 including in many cases those which are characteristic of glass. Some of 

 the rocks at this stage are light gray to green and exceedingly tough. 

 Many of these break with a ringing sound almost like phonolites. At a 



