PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 155 



Apobsklian.] 



No. 67. APOBSIDIAN. 



Near London [East Duluth]. Forms the point that is next west of the larger creek; continues thirty-five 

 or forty rods, and is subjected to great upheaval and pressure. 



Kef. Annual Report, ix, page 21. Annual Report, x, page 141. American Association for the Advance- 

 ment of Science, vol. xxx, page 164. 



Mc<j. A light red or pinkish specked rock, fine groundmass and some porphyritic 

 and amygdaloidal tendency, appearing as if a siliceous and shaly rock in masses had 

 been embraced in it. Some of these fragments weather dark green, and some 

 purplish red, or fawn color. It also has nests of calcite accompanied by fluorite. It 

 appears like a confused, half-baked, pudding-stone-like rock, or flow breccia. Evi- 

 dently a continuation of Nos. 65 and 66. 



Mic. The groundmass of the rock is reddened by haimlitc, and hardened by 

 poikilitic /ji/tnlz, both being secondary products after (jinx*. These, with much 

 calcite, are the only identifiable minerals. The quartz is sometimes grouped in 

 interlocking clear grains in what may have been originally vesicles in the glassy 

 mass, but in general it spreads through the whole slide, making a firm rock. 



AIJC. Cabotian. N. H. w. 



No. 68. APOBSIDIAN. 



PLATE I, FIGURE 3. 



Near London, just east of Duluth. A thin-bedded, red or pinkish, hard, condition of No. 67. 



Ref. Annual Report, ix, page 21. Annual Report, xiii, pages 100 (No. 152), 102.' Bulletin viii, page xxxiii. 



Mi'ij. A brown aphanitic rock, with small whitish to pinkish areas which 

 represent porphyritic feldspars. The rock has a laminated appearance and readily 

 splits along parallel cracks, in places splitting into leaves less than one-sixteenth of 

 an inch in thickness. Between the laminas are very frequently layers or veinlets of 

 quartz, sometimes of almost microscopic width, and sometimes nearly one thirty- 

 second of an inch in width. On a weathered surface these lamina? produce the 

 appearance of flow structure in a lava. 



Mic. The characteristic feature of the sections is the presence of large, irregu- 

 larly outlined areas of quartz which contain the other materials of the rock poiki- 

 litically. The whole rock is made up of these poikilitic areas and furnishes a 

 most excellent example of the micropoikilitic structure which is so common 

 in the acid rocks of the Cabotian. Scattered all through the quartz are minute 

 grains and specks which, even under a high power, are not clearly shown, of 

 magnetite, hematite and a grayish, almost opaque, substance. The latter is brought 

 out more clearly in polarized light, when it appears isotropic and gives to the section 

 a blotchy " pepper and salt " appearance. Although not now determinable, it seems 

 probable that this substance represents feldspar, now much altered, as is usual with 

 the feldspar of these rocks. In ordinary light the section shows irregular blotches 

 darker than the mass of the rock; these are areas more rich in the iron ores. 



