104 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



observed in the correspondiug' nouporphyritic basalts, the microophitic, inter- 

 sertal, navitic, pilotaxitic, and hyalopilitic. The vaiious basalts are connected 

 by transition phases. The close connection between the different varieties 

 is well shown where one passes from the fine-grained amygdaloidal rock 

 throug-h the fine-grained nonamygdaloidal over to the porphja-itic macro- 

 scopicalh" nonamygdaloidal type. 



petrographicai characters. — As Stated in tlic general description, these rocks do 

 not differ essentially from the nouporphyritic basalts just described. The 

 most important difference is in the presence of the feldspar phenocrysts, 

 giving them a porphyritic texture. 



Measurements upon the phenocrysts, made against the albite twinning 

 plane on zone _L to 010, according to the Michel L^vy method, give an 

 averag'e extinction angle of about 18°, which points toward its character 

 as labradorite. However, angles obtained lower than this indicate the 

 possibility of the association of andesine with the predominant labradorite. 

 The feldspars show the usual alteration products. One very infrequently 

 finds augite phenocrysts which have been completely uralitized associated 

 with the feldspars. Other phenocrysts are now represented by masses of 

 chlorite, with or withovit epidote, evidently pointing toward the basic and 

 mao-nesiau nature of the orig-inal mineral. As uralite is the common sec- 

 ondary product of pyroxene in these volcanics, the altered phenocrysts 

 represented b}" chlorite masses are not believed to have been pyroxene. 

 The original mineral was perhaps olivine. The very noticeable scarcity of 

 augite phenocrj^sts in the basalts stamps them as different from the great 

 majority of basaltic rocks and as being very similar to the basalts described 

 by Judd,^ from the Brito-Icelandic petrographicai province, in which por- 

 phyritic crystals of augite are seldom if ever seen and in which the j^heno- 

 crysts are feldspar and sometimes olivine. 



The groundmass in which the phenocrysts lie have generally the same 

 mineralogical composition and texture as the nouporphyritic rocks already 

 described, and the two kinds are supposed to have been originally similar.^ 



'On the gabbrus, dolerites, .and basalts of Tertiary age in Scotland and Ireland, by J. W. Judd: 

 Quart. Jour. Geol. Soc, Vol. XLII, 1886, p. 79. 



-The groundmass of one of these porphyritic forms differs somewhat in one important respect. 

 In it were observed numerous round areas of small size occupied by a clear white aggreg.ate, polariz- 

 ing in low gray colors. The centers of some of the areas were occupied by clumps of yellow grains, 

 with here and there a minute flake of chlorite. Others contain ouly tlie white material, which is 

 apparently secondary. The round areas are not sharply delimited, and hence are most probably not 

 microatnygdules. Their general appearance is strikingly like that of leucite in those plagioclase 



