PETEOGEAPHICAL CHAEAOTERS OF GEOVBLAND FOEMATION. 449 



be said that the formation contains less iron than in the Felch Motintain 

 range, and consequently the lighter-colored varieties are more abundant, 

 that it contains more detrital material, and that in the Michigamme Moun- 

 tain area the texture is generally closer and less granular. Moreover, in 

 passing north from the Michigamme Mountain area to the Fence River area 

 we find at the Sholdeis and Doane explorations that the lower portion of 

 the formation is composed of ferruginous quartzite, which is succeeded 

 higher up by actinolite-schists and griinerite-schists similar in all respects 

 to the characteristic rocks of the Negaunee iron formation in the western 

 Marquette district. In this change in character as the Marquette district is 

 approached is found the lithological support for the viewj first suggested by 

 the distribution of the lines of magnetic attraction, that the Groveland 

 formation is the southern equivalent of both the Ajibik quartzite and the 

 Negaunee formation of the Marquette district. The passage to a more 

 crystalline condition in going from south to north is also in accord with the 

 like changes already noted in the lower formations. 



Under the microscope the close texture of the Grroveland rocks of 

 Michigamme Mountain is seen to be due to the minuteness of the quartz 

 grains of the groundmass, and to the abundance therein of chalcedony. 

 The coarse quartz grains are all detrital and are often beautifully enlarged. 

 In many slides feldspar pebbles occur, and in many also sericite and chlorite 

 are prominent in the groundmass. The iron oxides, including both mag- 

 netite and hematite, in single crystals and also in aggregates, are well dis- 

 tributed, as in the Felch Mountain sections. A similar grouping of these 

 in round pebble-like areas as in the Felch Mountain range is also beauti- 

 fully shown. In one slide the matrix is a rhombohedral carbonate, prob- 

 ably calcite, in which are embedded quartz g-rains and the iron ores in 

 single crystals and iiTegular aggregates. 



The most interesting features of the thin sections from Michigamme 

 Mountain are the pressure effects. In man}^ slides the detrital quartz 

 grains are strained to an extraordinary degree. In one case the stage was 

 rotated 45° before the black wave of extinction com^Dletely traversed a 

 little pebble 0.3 mm. in diametei". Almost every section is ci'ossed in sev- 

 eral directions by fractures healed by the deposition of coarse quartz 

 and the iron oxides. 



MON XXXVI 29 



