Plate XX. — Prom the Upper Horizon of the Qitaetz-Slate Member. 



Fm. 1. Quartzitc. Si>pciiuen 9082, sIuIp 2780. I'mni the east side of Soc. 19, T. 45 N., K. 1 E., 

 Wisfonsii). lu ordinary light, X 25. The rock is a vitreous quartzite, and .vet the tigure 

 shows with perfect distinctne.ss the outlines of each of the rounded grains of sand just 

 as they were originally deposited. (See p. 160.) 



Fig. 2. The same, in polarized light. The causi' of the ])resent strong and vitreous cliaraeter of the 

 quartzite is clearly shown by this figure. Each of the grains of quartz has added to itself 

 other ipiartz uutil the grains have met and interlocked. This is a fine instance of the 

 induration of a sandstone by simple eulargcuK ut of the original grains. (See p. 160.) 



Fig. .H. Ferrugiuous quartzite. Specimen 91.')4, slide 2804. From the .S\V. i of Sec. 27, T. 46 N., R. 

 2 E., Wisconsin. In polarized light, X 25. Tlie rock is again a vitreous quartzite, whicli 

 has, however, a brown color. Tlie outlines of the original grains are distinctly seen as 

 in Fig. 1. The cohn- of the rock is due to tlic oxide of iron located in the interstices. (See 

 p. 162.) 



Fig. 4. The same, in polarized light. As in Fig. 2, each of the rounded grains of quartz has been 

 enlarged. The interlocking of these enlargements has been interfered with by the included 

 iron oxide, so that the rock is not so vitreous and strong as that from which Figs. 1 and 2 

 are taken. (See p. 162.) 



488 



