984 THE GEOLOGY OF MINNESOTA. 



(Petrographic and petrologic. 



schist or biotite gneiss (Fifteenth Annual Report, p. 351, and rock No. 983). In all 

 cases, even when a normal gabbro has resulted, the different minerals have a roundish 

 habit, as if cotemporaneously developed. The ophitic structure prevails in those 

 cases where there was a late consolidation or a second generation of augite, and this 

 occurs especially in those masses that have moved more or less from their birth- 

 places, /. e., in the diabases. 



There is no single petrographic character that is unique in its mode or geo- 

 graphic place of occurrence within the whole zone, ranging from partially metamor- 

 phosed clastic greenstone to typical gabbro, and even to diabase; but the mineralogic 

 composition varies according to some unknown law, or no law, and the contrasting 

 structures often blend in one rock or are associated with minerals which usually are 

 considered divorced from them. This singular rock, or series of rocks, included under 

 the term muscovadyte,* seems to be explicable only on the hypothesis that a variable 

 clastic, though comparatively basic in composition, was subjected to a variable meta- 

 morphosing force, the resultant rock being determined by the depth at which the force 

 was applied, the amount of pressure and moisture, the degree of heat and the propor- 

 tionate amounts of the chemical elements available from place to place for the pro- 

 duction of the minerals which now are found in the rock. If at any place any of 

 the oldest (originally massive) greenstones were involved in this metamorphism 

 and refusion, it is probable that the resultant rock would be some of the more basic 

 phases of the general gabbro mass. 



In addition to the foregoing general statements as to the confused petrographic 

 characters of the petrologic zone intervening between the Archean greenstones and 

 the Taconic gabbro, there is need perhaps of some specific, recognizable case to which 

 all geologists can be referred tending both to centralize the argument and to eluci- 

 date the diversified phenomena. Only referring, here, to the statements made in 

 volume iv, pages 303, 304, respecting the transition studied at Disappointment lake, 

 more direct attention will be called to Tock No. S47G, already referred to.f This 

 came from a definite locality and from a rock which has definite relations to a coarse 

 gabbro. It is from the angular masses represented as embraced non-conformably 

 by gabbro in the figure below (figure 56). This figure was drawn from nature by 

 Dr. A. Winchell in 1886, at Gabemichigama lake. The structural relations are 

 described by Dr. Grant, and have already been quoted (page 982). By both this rock 

 was styled muscovadyte, and by the writer it has been described as muscovadyte 

 (page 905), from two sections that have been examined. In the first section it 

 appeared as an ophitic (diabasic) rock. In the second, along with but imperfect 



*Two intcrcstiii!,' extremes of the family of metamorphic rocks derived from the clastic j-reenstonrs arc represented by 

 Nos. 399H and 406H. 



tThe same structural evidence ia presented by rocks Nos. 1287 and 128ft, at Mayhew lake, the former beinx a aramilitir 

 gabbro unconformable below coarse gabbro. 



