BASIC VOLCANIOS OF HEMLOCK FORMATION. 99 



mass. Where weathered, the rocks are usually covered by a thin crust, in 

 which gray, brown, and pinkish tints prevail. 



The rocks vary in texture very much, from the dense aphanitic kinds 

 to medium fine-grained varieties. The latter are usually less amygdaloidal 

 than are the aphanitic forms, and approach in appearance both macroscopic- 

 ally and microscopically the coarser-grained basalts or dolerites represented 

 in the Michigamme district by the coarse-grained intrusives. Owing to the 

 basic nature of the rocks, they have generally suffered much alteration, and 

 as a result the original texture is in many cases poorly jjreserved. On the 

 whole, however, it is remarkable, considering their age and basic character, 

 how well preserved it is. Where it is preserved it varies from the micro- 

 ophitic to the various microlitic textures, such as intersertal, navitic, pilo- 

 taxitic, and hyalopilitic, and lastly glassy. In places a flowage structure is 

 beautifully brought out by the position of the feldspar microlites, especially 

 around the amygdules. 



The constituents present are plagioclase, light-green fibrous hornblende, 

 epidote-zoisite, chlorite, calcite, muscovite, apatite, sphene, quartz, magnet- 

 ite, and pyrite. Of these the feldspar, apatite, and iron oxide alone are 

 original. In some places the hornblende is wanting, the chlorite then 

 appearing in correspondingly greater quantity. 



The feldspar ordinarily occurs in lath-shaped crystals showing twins of 

 the albite type, but where the texture is fine the feldspars are microlitic, and, 

 while showing their prominent long extension, the edges of the various crys- 

 tals interfere, and the outlines consequently are less sharp. 



In some of the rocks which appear to have been vitreous the feldspar 

 forms feather and sheaf like aggregates (figs. A, B, PI. XXVI), apparently 

 quite similar to those described by Ransome in rocks from Point Bonita, 

 California.-' No reliable measurements could be made upon the microlites, 

 and consequently their character could not be determined. The feldspar is 

 more or less completely altered to aggregates of epidote-zoisite which have 

 chlorite associated with them or are altered to sericite. In a number of 

 places minute limpid spots of secondary quartz and albite are present. The 

 very small quantity of apatite present shows its usual characters. Titanif- 

 erous magnetite ore is apparently the only oxide present. It occurs in crys- 

 tals and in irregular grains, which in a few cases are not entirely altered, 



' Eruptive rocks of Point Bonita, by F. Leslie Ransome : Bull. Univ. of Cal., Vol. 1, 1893, p. 84, fig. 6. 



