PETROGliAPHICAL CHARACTER OF AJIBIK QUARTZITE. 291 



ferruginous chert and jasper veins often have the iron oxide and the (juartz 

 arranged in bands or irregularly distributed, and the veins are exactly similar 

 to the jaspilite of the Negaunee formation. Often the vein material is mingled 

 with fragraeutal quartz, the grains having been broken from the rock and 

 fallen in the crevices. Where the individual grains of the rock were sun- 

 dered, the parts were cemented by the secondary quartz and iron oxide exactly 

 as were the larger spaces. The recognizable original grains of quartz show 

 strong dynamic effects, all of them giving undulatory extinction, and many 

 of them being broken into several individuals, or even wholly granulated. 

 In some cases the cracks are in two sets at right angles to each other, the 

 cracks of each set having a parallel arrangement. The areas in whicli the 

 grains were rent asunder and those in which they were not are very irreg- 

 ular, and in the field the first are usually separated from the second by 

 stains of iron oxide. In those cases in which the secondary (|uartz is 

 abundant and the primary quartz was granulated, so that it no longer has a 

 clastic appearance, we have an intricately interlocking mass of quartz grains 

 of various sizes in which the original material can not be discriminated 

 from that which has come in later. In some places the whole rock is com- 

 posed of small, closely fitting granules of quartz. The granulated material 

 is commonlv finer or coarser than that of the interlocking and intersecting 

 veins, and in the latter iron oxide is usually abundant. These rocks, in 

 which the evidence of frag-mental origin has disappeared, and yet which do 

 not have a schistose structure, are called quartz-rocks. All of these jihases 

 are so similar to the jaspilite of the Negaunee formation that the two could 

 not be separated in thin section. However, these extremely altered rocks 

 are traced into those which are less modified, there first appearing a few 

 distinctly clastic grains, then clusters of them, until we have an intermediate 

 variety in which perhaps half of the secti<m shows fragmental quartz buried 

 in a crystalline matrix. 



Resulting from the difiering modifications of the original sandstone, 

 we therefore have in the formation quartzite, cherty quartzite, ferruginous 

 quartzite, ferruginous cherty quartzite, (juartz-rocks, quartzite-breccia, A'ein 

 quartz, vein chert and jasper, and other phases. 



The rather peculiar autoclastic rocks which resemble quartzite-conglom- 

 erates were mentioned in the macroscopical description. The pebble-like 



