70 THE GEOLOGY OP MINNESOTA. 



[Peripheral phases of the gabbro. 



it is a magnetite iron ore, frequently, but not always titaniferous. In both there are 

 still small amounts of the other minerals. 



These minerals are all secondary in the sense that they have resulted from the 

 metamorphism of some others. This can be inferred not only from the facts to be 

 seen in the field but also from a study of the minerals themselves with the micro- 

 scope. It appears, therefore, that there was a prior rock whose profound alteration 

 has generated all these varieties. It appears also that such rock while being one 

 that could furnish, for the most part, the phase known as granulitic gabbro, or mus- 

 covadyte, was sometimes capable of furnishing a large amount of quartz, or again of 

 quartz and magnetite. No rock is known having such qualities and such variations 

 except the clastic greenstones. They were mainly a basic effusive, as already shown in 

 discussing the Archean, but they vary by being fragmental, acquire quartz, which 

 was chemically precipitated, or was added mechanically, hold the jaspilyte lodes, 

 and sometimes are coarsely conglomeratic. The minerals of the intermediate rock 

 and its structures, which stand between greenstone and gabbro, can all be produced 

 by reforming the greenstones under conditions not allowing complete liquidity. The 

 agent was not heat alone, and the condition of the magma was not that of simple 

 dry fusion, but that of aqueo-igneous transformation. In this process there is not 

 believed to have been any important transpositions of the original elements from 

 their original places. The change that passed over the original rock when the result 

 came to be the muscovadyte was a chemical reestablishment of the old minerals 

 native to the original greenstones, but lost through decay and age, or of their congeners. 



If the nature and origin Of the Archean greenstones be recalled for a moment, 

 it will appear how well suited to the production of such a rock they were, and espe- 

 cially how easily, on metamorphism, they could produce all the peculiar phenomena 

 seen in the association of olivine and quartz with labradorite, hypersthene, etc., in 

 the iron-bearing muscovadyte. The tuff aceous elements, falling into the ocean from 

 volcanic ejection, the quartz and magnetite as limonite falling at the same time or 

 interruptedly with the volcanic ash from chemical precipitation, gave origin to a 

 stratified greenstone, or green schist, in which were included the iron bodies now 

 known as jaspilyte. The basic layers associated with the vitreous Pewabic quartzyte 

 were "tuft'aceous eruptive fragmental elements" of cotemporary date with the 

 quartzyte in its original condition. 



These muscovadyte phases are to be considered as specialized conditions of the 

 old greenstones, but complex and nondifferentiated stages of the gabbro due to excep- 

 tional conditions which arrested complete regeneration. We here reach the most 

 interesting, and perhaps the most important, part of this topic, viz.: 



The origin of 1h<> yabbro. In the foregoing only the greenstone alliance of the 

 muscovadyte has been presented. The investigations of the officers of the survey 



