BASIC DIKES IX THE NOIITIIEKX COMPLEX. l^l 



pseuil<)iii(>r|)lis <if the latter luiiicral, iviilnciiiij;- it cniuitlctch- in some cases. 

 In other eases bars of tlie oi)a(iiie iron oxide form a network in wliose 

 meshes are the white o[)a(|ne grains of lencoxene. In tlie thieker aoyre- 

 gates the lencoxene i)asses into a dense yellowish-l)rown mass witli tlie 

 pleocln-oism of sjiliene. 



The character of the alteration that chanf>-ed diabases into epidioi-ites, 

 coupled with the existence of schistosity in the latter rocks, is thouo-ht to 

 be sufKcient reason for a.scribin;^- the origin of the epidiorites to dvnamo- 

 metainorjihism. 



The "diorites"of the Northern Complex are ))rol)ablv altered diabases 

 in which the new hornl)len(le has assumed a compact rather than a til)rous 

 form. The hornl^lende crystals in these rocks are always frayed out at 

 their ends, but in cross-section they are compact and idionior])hic. The 

 original feldspar has been entirely replaced l)y a transparent plagioclase 

 that is cut through and through by slender needles of hornldende, at wliose 

 ends terminal planes may often be detected. Under crossed nicols large 

 areas of the plagioclase break up into main' small ones, interlocking with 

 one another by sutures which follow the most intricate courses. This feld- 

 spathic mass dififers so greatly from that of the epidiorites in appearance, in 

 freshness, and in its structure that we must regard it as essentially different 

 in origin. The feldspar of the epidiorites is a decomposed jilagioclase, 

 while that of the diorites is apparently a recrystallized one. The diorite 

 as it now exists is not an original rock. It has been formed from some 

 preexisting eruptive, but whether from an original diorite or a diabase is 

 not certainly known. 



The final products of weathering of all the Ijasic rocks de.scribed as 

 occtirring in dikes are chlorite, epidote, kaolin, calcite, and (piartz. A 

 number of dike masses are known in the Northern Complex that consist 

 princijially of these minerals. Most of them present the ophitic texture of 

 diabases, while in others the granitic texture of diorites and gabbros is 

 recognizable. All are highly schistose. Chlorite h by far the most 

 abundant component in them, and the rocks therefore are practically 

 chlorite-schists. All that have been studied are unquestionably squeezed 

 eruptives. 



