GROVELAND FORMATIOIT TN PELGH MOUNTAIN RANGE. 421 



exceeding a millimeter in diameter. These oval forms are confined to the 

 more siliceous bands, and are inuch more distinct in some of the slides than 

 in others. Often the outlines are reenforced by rims of closely set mag- 

 netite iudi^-iduals, somewhat coarser than the dust-like crystals within. The 

 long diameters of adjacent ovals are parallel to one another and to the band 

 in which they lie, and are often closely j^acked like pebbles. Occasionally 

 the little grains of iron ore within the ovals have a distinctly concentric 

 arrangement. 



Between crossed nicols these areas are seen to have had in some 

 instances a distinct influence on the crystallization of the secondary quartz. 

 When they are large and closely packed, each oval includes a large number 

 of interlocking quartz grains, and occasionally in such cases there is some 

 difference in size between the quartz grains inside and those outside the 

 ovals. In the triangular and quadrangular areas lying between the larger 

 ovals, and bounded by curving segments of their perimeters, the secondary 

 quartzes are frequently larger than those within, and are placed normal to 

 the boundaries, precisely as if they had grown outward from the ovals into 

 free spaces. Often, however, a single individual of secondary quartz lies 

 partly within and partly without the oval. On the other hand, when the 

 ovals are small, one or more may be completely or partially inclosed within 

 a sing'le quartz individual. The interlocking quartz grains within the large 

 ovals show no indications of having formed in open spaces, even when the 

 included iron ores have a tendency, as occasionall}^ happens, to a concentric 

 arrangement. The faulting and brecciation so plainly seen in many of the 

 thin sections have also displaced and separated the oval areas. It seems 

 perfectly clear to me that these forms represent a structure originally 

 possessed by the rock from which the various phases of the iron formation 

 have been derived, and which has been preserved through the subsequent 

 metamorphism. 



From the facts described above, it is evident that the Grroveland for- 

 mation is made up of highly metamorphic rocks, which still, however, retain 

 some original clastic material as well as certain original structural characters. 

 With the exception of the rather rare clastic grains of quartz, titauite, 

 feldspar, etc., the minerals which now chiefly compose these rocks — namely, 

 quartz and the crystalline iron oxides — are not clastic, but have crystallized 

 in place. It is a matter of great interest, therefore, to determine, if possible, 



