84 THE GEOLOGY OP MINNESOTA. 



[Qabbro. 



The greenish alteration product from the pyroxene is, however, not always charac- 

 terizable as amphibole. It is fibrous when the alteration is complete as amphibole, 

 or granulo-fibrous when it is not far advanced. The change also proceeds to the 

 production of a chlorite having the blue color and pleocroism otpennine. 



There is also seen, rarely, another stage, a micaceous one, between amphibole 

 and chlorite. It is hardly distinguishable, since, in all cases, so far as observed, the 

 mica produced is so far permeated with chloride characters, both in internal struc- 

 ture and in optical aspect, that it is intermediate in ensemble between chlorite and 

 mica. When the micaceous characters remain they are expressed by higher double 

 refraction and a more widespread uniform darkening at certain angles over the whole 

 area of the grain or by evident cleavage. When the change is so far perfect as to 

 constitute a chloritic element, the polarization is in shades of blue and yellow which 

 supplement each other in rotation, each in the form of bright filaments, and the 

 grain never assumes the darkness which is brought out in the micaceous stage. In 

 ordinary transmitted light the micaceous grains are clear and structureless, or show 

 a crooked cleavage, giving little indication of the fine, confused internal structure 

 which the polarization colors express. The chlorite grains are continuously dark 

 between crossed nicols when the section is parallel to their cleavages. 



The pyroxene of No. 1 is further described under No. 1C. Magnetite appears 

 both as apparently original and as secondary products. In the former state it con- 

 stitutes an important ingredient in the composition of the rock, and as a secondary 

 product it seems to have originated in an early stage of the rock's history, probably 

 before it had become cool. Indeed, it is reasonable to suppose that many of the 

 changes to which the various minerals have been subjected were completed prior to 

 cooling. In the case of the magnetite, whether of the original minerals that segre- 

 gated from the magma, or of those that arose from chemical rearrangement after con- 

 solidation, there seems to be no essential difference in its visible characters. The 

 original grains are sometimes two or more millimeters in cross section, and occupy 

 original positions amongst the larger grains, while the secondary grains are of micro- 

 scopic size, and lie in the cleavage cracks of the labradorite and at the angles of the 

 pyroxenes. Magnetite being normally one of the earliest of the original minerals to 

 take its place in the consolidating magma, it is yet possible that the original and 

 secondary grains, as here distinguished, did not differ widely as to the time of their 

 formation. In any case the magnetite, even in its finest grains, can hardly be con- 

 sidered a product of ordinary weathering, nor of dynamic action, and the intimate 

 relations subsisting between all the grains, especially the approximate pseudomor- 

 phism after pyroxene, indicate that some of the larger grains are likewise of secondary 

 origin. 



