1008 THE GEOLOGY OF MINNESOTA. 



DESCRIPTION OF PLATKS. 



PLATE I. 



Page. 



FIGURE 1 Diabase with finely lustre-mottled or ophitic structure, and remnants of 



the glassy magma. Rock No. 100, about "f 173 



FIG. 2 Diabase, more coarsely ophitic. Rock No. 108, about 2 T 6 177 



FIG. 3 Apobsidian, showing a micro-poikilitic structure of the secondary quartz. 



Rock No. 68, about Y. Compare Nos. 129, 647 155 



FIG. 4 Diabase (?) or granulitic gabbro(?) showing a poikilitic structure of the 



later labradorites. Rock No. 122, about \ 6 186 



5 Aporhyolyte, showing perlitic structure perpendicular to the fluidal lamina- 

 tion,, quartz phenocrysts and embayments. Rock No. 140(1), about ' T . . . . 212 

 6 Aporhyolyte, showing perlitic structure, with anorthoclase (010) and embay - 



ment at one corner. Rock No. 140 (7), about 2 , 5 216 



FIG. 7 Gabbro, showing olivine having poikilitic structure. Rock No. 258, about s , 5 



FiG. 8 Taconyte, showing a grouping of magnetite dust particles suggesting the 



cells of Foraminif era. Rock No. 441 , about Y 368 



p IG . 9 Aporhyolyte, showing minute remnants of feldspar in identical orientation. 



Rock No. 520, about V 396 



FIG. 10 Thomsonite in coarse fibres piercing a network of prehnite, cut parallel 



and transverse to the elongation. Rock No. 573, about 2 , 5 431 



FIG. 11 Granite, showing an embayment in quartz, filled with imperfectly spheru- 

 litic magma continuously united with a similar but more perfectly 

 spherulitic structure which surrounds the phenocryst. Rock No. 619, 

 about V 452 



FIG. 12 Diabase, showing a single ophitic augite embracing 25 or more microliths 



of feldspar. Rock No. 820, about V 559 



