PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 739 



Taoonyte.] 



Age. Pokegama (top of the quartzyte). 



Remark. Nos. 1530 and 1294, from opposite ends of the known Mesabi Iron 

 range, are remarkable phases of the taconyte, and are alike in their remarkable 

 characters. Several photographs are shown on plate III, viz.: figure 4 from No. 

 1294, shows the general oolitic structure in its best condition; figure 5, from the 

 same rock, shows the same when permeated and almost lost by the accumulation of 

 iron ore; figure 6, from No. 1294, shows the amorphous nucleus which, having been 

 once surrounded by the oolitic concentric bands, was broken and again surrounded 

 by similar concentric bands non-conformable with the former. Such irregularities 

 are common and show that this structure did not grow up within this rock since 

 consolidation, but pertained to these grains prior to their lodgment in this mass. 

 Figure 7, from No. 1294, shows the non-conformity of the nuclei with the encircling 

 bands, proving the earlier existence of the nuclei. This figure shows, also, an 

 irregular streamed structure in the nuclei and a passage of this into the con- 

 spicuous peripheral banding. This, with other similar gradations from one to 

 the other, indicates that the internal structure of the nuclei, although usually amor- 

 phous, might sometimes assume an imperfect banding, having perhaps the same 

 origin. Figure 8, from No. 1530, shows the general aspect. At the centre of the 

 photograph is a grain which is shown magnified in figure 9. In this grain the nucleus 

 is different from any other seen in either No. 1294 or No. 1530. It is distinctly a 

 fragmental compound nucleus. It embraces eleven main parts, and three of them 

 are of angular apparently earlier quartz, the others being of flint (or devitrified glass) 

 and of coarser interlocking taconitic quartz of varying size of grain. This shows 

 that the "tacouitic" structure of the rock taconyte, so far as it involves the fine 

 interlocking quartz, sometimes was formed before the rock taconyte, as now ordina- 

 rily known, was deposited; and therefore, that within the individual, rounded, pebbly 

 grains of the taconyte, the interlocking quartz may have developed earlier than in the 

 mass of the taconyte, and may have been in each, pebbly grain dependent on a pri- 

 mordial condition of the pebble itself. This different development of the interlocking 

 quartzes, in the different rounded pebbles, even in the pebbles of the nucleus shown 

 in figure 9, may not, however, have been due to difference of time, but the general 

 silicification may have been all simultaneous throughout the rock, and the varying 

 grain of the interlocking quartz may then have been due to this primordial difference 

 of the original pebbly masses. Figure 10, from No. 1530, shows the encircling bands 

 about an amorphous but ferruginated nucleus, broken and displaced. This also 

 shows that these encircling bands were formed before the pebbles acquired their 

 present positions. Some of the pebbles cracked irregularly, apparently by shrinkage, 

 after the formation of the encircling bands, and in some cases they seem to have 



