928 THE GEOLOGY OP MINNESOTA. 



[Taconyte. 



the change only about their margins. As to the nature of these radiating fibres, they 

 are plainly adinolite, or an amphibole allied. At the present stage of change shown 

 by this rock, about five-sixths of the whole area are of this fibrous network and the 

 yellow substance that gives place to it, the rest being of the interlocking quartz, which 

 forms the general matrix. One section. 



Age. Animikie (iron-bearing member). 



Remark. This slide shows that the green substance; supposed by Mr. Spurr to 

 be glauconite, is of such a nature that it almost spontaneously gives place to an 

 amphibole and that it thus has an alliance with the darker debris of the tuffaceous 

 greenstones. 



The figure above is designed to illustrate some of the changes that take place in 

 this primordial greensand. Within the area of the field is one grain of that sub- 

 stance so magnified as to occupy nearly all the space, that is, about one hundred 

 diameters. Some grains in the rock are larger, and their size also runs down to mere 

 specks. There are to be noted various things of interest and importance in connec- 

 tion with this greensand: 



1. In its earliest state, so far as can be affirmed by the evidence of the section 

 illustrated, it is feebly translucent, greenish, or olive green, and is broken into many 

 irregular areas by cracks that cross it in the manner of shrinkage cracks. 



2. It becomes transparent when it begins to crystallize by the chemical sepa- 

 ration of its ingredients into definite compounds. Several such more transparent 

 roundish spots are seen in the figure. These are, for the most part, filled with quartz 

 in imperfect or globular fine grains, but are also somewhat occupied by actinolite. 

 The size of the quartz grains in these confined areas is smaller than that of the quartz 

 which is outside of the glauconite, and which forms the groundmass of the rock. 



3. Radiating, fibrous actinolite (or perhaps cummingtonite) pierces the quartz, 

 being apparently cotemporary with or a little earlier than the quartz. This amphi- 

 bole surrounds the greensand grain and its fibres run independently into the margin 

 of the grain in such a manner as to show that it, as well as the quartz, originated 

 wholly from the greensand. Such actinolite spicules are sometimes seen (in other 

 greensand grains) wholly within the body of the greensand. 



4. The greensand begins to crystallize and is wholly transformed about its 

 margin and at points within the grain, giving rise to quartz and actinolite (or cum- 

 mingtonite). 



5. In the case of the grain illustrated there is no apparent segregation of iron 

 ore, but in other slides cut from this rock sample iron ore is manifested as one of 

 the secretions from the greensand. These are its appearance within certain grains, 

 rendering them wholly opaque, and in the same way as with the quartz and cum- 



