BASIC DIKES IN THE NORTHEKN COMPLEX. 179 



the latter had become foHated, and must be younger than the diorites and 

 epidiorites, all of which are schistose. 



In texture the diabases may be very coarse grained, very fine grained, 

 or they may contain some glass. Miueralogically they jiresent no special 

 features. Most of them are nonolivinitic ; a few contain pseudomorplis of 

 chlorite and limonite after olivine. 



An example of one of tlie freshest of the coarse diabases is found in a 

 dike 75 feet in width cutting granite at 1,230 steps N., 450 steps W. of the 

 SE. corner of sec. 23, T. 48 N., R. 28 W. (Atlas Sheet XXI). So coarsely 

 granular is it that the rock approaches a gabbro iu structure, though the 

 augite is younger than the plagioclase and fills the interstices between the 

 laths of this mineral. Tlie components of the rock are magnetite or ilmen- 

 ite, apatite, labradorite, and pyroxene, besides various alteration products 

 of the two last-named minei-als. The fresh pyroxene has the jjink tint so 

 common to the monoclinic pj^roxene of Lake Superior rocks. On its edges 

 it is altered to fibers of light-green hornblende, with which are interspersed 

 a few grains of magnetite. The plagioclase is mainly well preser^^ed. Its 

 twinning lamellae are broad and clearly defined, and the symmetrical extinc- 

 tion on each side of the twinning line is about 21°. In the small areas 

 between the most altered pyroxene grains the feldspar is decomposed. It' 

 is reddened by cloudy secondary substance, and is filled with chlorite and 

 amphibole flakes and needles. In these portions of the slides the largest 

 apatites are to be found. 



Sections of other fresh granular dikes present nearly the same phe- 

 nomena as those above described. In most of them the diabasic structure 

 is very pronounced and the grain is finer than in the case of the dike last 

 mentioned. Moreover, in nearly all, alteration has progressed a little farther. 

 Green biotite and brown hornblende in small quantities are nearly alwa^'s 

 the accompaniments of the green hornblendic and chloritic decomposition 

 l)roducts of the pyroxene. They occur in small flakes on what were the 

 peripheries of the original pyroxene areas, and no doubt owe their origin 

 partly to the feldspar. Upon further alteration the l)rowu hornblende 

 passes into a chlorite, while the green miea retains its j)roperties. 



