196 THE GEOLOGY OF MINNESOTA. 



[Apobsidian. 



(see general section of the rocks of this vicinity in Part III). They both contain 

 considerable quartz. 



Mirrv.sc<jj>im/li/ they are not the same rock. One is an ophitic brown diabase, 

 in which many grains of quartz, of angular forms, are scattered through the whole, 

 lying between the plagioclases and the magnetite. The plagioclase is so decayed 

 that it cannot be determined any closer. It is in lath-shaped microliths, brownish 

 red with lienmtiti", also, between crossed nicols, specked with numerous minute 

 transparent inclusions, which are apparently in part calcite, and muscovite, and in 

 part quartz. No augite is visible, and some minute grains which polarize so feebly 

 as to be almost dark between crossed nicols, surrounded by opaque rims, are probably 

 the remains of original olirinc. Maf/iiefitc is abundant, probably after augite. This 

 rock retains its evident ophitic structure. All its original minerals are so far gone 

 that only the plagioclase can be identified with certainty by its form. 



The other rock is very different. It consists almost wholly of secondary quartz, 

 but this quartz is so charged with other substances that it is nearly or quite opaque. 

 While some of the grains are quite small, others are of considerable (microscopic) 

 size. They are all so filled with magnetite and other much finer impurities, which 

 are unidentifiable, that between crossed nicols the section is semi-dark. This quartz 

 has not its original form, but has been rearranged by contact with the other rock. 

 It has a spongy, sometimes even an ophitic, manner of enclosing the other substances, 

 evidently being subsequent to them. Each siliceous area darkens independently and 

 entirely, though often divided into several independent grains. In some places an 

 imperfect, radiated, spherulitic structure is evident. This is shown by the occurrence 

 of a permanent black cross, which, as the stage is revolved about the point of crossing 

 of the threads, retains its arms constantly parallel to the threads. 



Throughout the quartz is another polarizing mineral which sometimes is indis- 

 tinctly pegmatitic with the quartz, extinguishing at positions not in unison with the 

 extinction of the quartz. It seems to be imperfectly developed crystallographically 

 and chemically, in that respect resembling the quartz, but not limpid. It is presumed 

 to be a feldspar, probably anorthoclase, judging from the nature of the feldspar in 

 the rocks associated with this in the same region, but it is impossible to determine 

 it microscopically. 



Four sections. 



AIJC. Cabotian; red-rock series. 



Remark. This rock is probably a phase of No. 129. It is non porphyritic, either 

 with quartz or feldspar, and every feature which appears under the microscope 

 would permit of its having resulted from devitrification of an original glass, after 

 solidification. Yet it may perhaps also be supposed that these micropegmatitic struct- 



