BONE LAKE CRYSTALLINE SCHISTS. 151 



schists. It occurs as a secondary constituent. It is found, however, as a 

 primary constituent in a few rocks which, as they still possess renmants of 

 original igneous textures, strictly speaking, should not perhaps be included 

 with the crystalline schists. They represent more properly the transition 

 stages to the crystalline schists, but the process of the alteration of the 

 feldspar is so well shown in these that it is considei'ed expedient to mention 

 it at this place. The original feldspar occurs in this transition phase in 

 the large tangled intergrowths commonly seen in andesitic and. basaltic 

 rocks, as individual phenocrysts, and as microlitic lath-shaped individuals 

 in the groundmass. The greatest interest centers in the phenocrysts, as in 

 them the changes which take place are more clearly seen. The feldspar 

 phenocrysts are always cloiady, due to numberless black ferruginous inclu- 

 sions. They also inclose the various secondary dark silicates composing 

 the groundmass, grains of epidote, flakes of biotite, and crystals of horn- 

 blende. These are usually surroimded by very narrow clear zones, appar- 

 ently feldspar. Near the edges of such altered crystals, and especially in 

 the more altered individuals, these inclusions are more numerous, and are 

 accompanied by grains of quartz and new feldspar (albite !). These last 

 two have certainly been derived from the alteration of the feldspar, but 

 that mineral may possiblj?- also have contributed something to the produc- 

 tion of the dark silicates. 



The secondary feldspar, that of the schists proper, is found in grains 

 usually unstriated, though in a few cases striations were observed. This 

 feldspar was not determined, but is probably albite The chlorite is in flakes 

 scattered through the schists, showing the usual characters. 



Epidote, muscovite, quartz, and rutile appear as usual. 



Ilmenite is present in one case as micaceous titanic iron oxide, and is 

 then in extremely thin plates which show a beautiful hexagonal develop- 

 ment, though more frequentl}'- the plates are rounded.' They are transparent 

 with the characteristic clove-brown color. The thicker plates are thin 

 enough to be transparent only along the edges. 



The iron oxides, magnetite, and hematite occur in some of these rocks 

 in large quantity. In certain parts of the area underlain by these schists 

 considerable excavations have been made in search of iron, the presence 

 of which was indicated by the magnetic needle, and moderately large 

 bodies of ore have been found, though in no case in sufficient quantity to 



