200 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



medium to coarse. Probabl j the most striking textural characteristic is the 

 pecuHar mottled appearance described as "hister-mottling" by Pumpell}',' 

 to which the name poikihtic^ has of late years been more generally applied. 

 This texture is almost always brought out macroscopically on the weathered 

 surfaces bv the difference in the ^^'eathering of the feldspar and the augite 

 or uralite, these being the prominent mineral constituents of the rock. 

 This Doikilitic texture is most common in the coarsest of the metadolerites. 

 In such rocks the augite or uralite areas are of large size, quite commonly 

 2 centimeters in diameter, and show their iiiottled character very plainly to 

 the naked eye. 



In the medium-grained dolerites the ordinary ophitic texture is pre- 

 dominant, though in these rocks poikilitic areas may be seen. In fact, these 

 poikilitic areas really possess an ophitic texture, according to the definition 

 of that texture b}- A. Michel L^vy, for the feldspars are developed as laths 

 and the pp-oxene is the mesostasis in which the feldspars lie.^ It is thus 

 clear that, restricting the statement to these dolerites, the ophitic texture is 

 at times included in the poikilitic, and that under such circumstances the two 

 can not be considered as totally different and independent textures, but are, 

 on the contrary, practically identical. 



In one dike of dolerite the influence which the conditions of consoli- 

 dation exert upon the texture is well shown. This dike is only 8 feet wide, 

 but the center is developed as a dolerite, while along- the edges where cool- 

 ing was more rapid, the rock is a porphyritic basalt. The porphyritic 

 texture is caused by the development of pyroxene and feldspar phenocrysts, 

 which lie in a dense basaltic groundmass. 



Microscopical. — The Original minerals of which the rocks were composed 

 were feldspar, C[uartz, pyroxene, olivine, biotite (!), apatite, and titano- 

 magnetite. The minerals which are now present in the rocks are for the 

 most part secondary. They are hornblende, muscovite, epidote-zoisite, 

 chlorite, biotite, sphene, leucoxene, calcite, albite, quartz, and iron pyrite. 

 Of these, hornblende is by far the most prominent constituent. A study of 

 the isolated specimens of these rocks might result in their determination as 



' Metasomatic development of the copper-bearing rocks of Lake Superior, by Raphael Pumpelly. 

 Proc. Am. Acad. Arts Sci., Vol. XIII, 1878, p. 260. 



^ On the use of the terms "poikilitic " and " micropoikilitic " in petrography, by G. H. Williams : 

 Jour. Geol., Vol. 1, 1891', pp. 176-179. 



^Structures et classilioation des roches eruptives, by A. Michel L6vy, Paris, 1890, p. 30. 



