216 THE GEOLOGY OF MINNESOTA. 



[Aporhyolyte. 



striated or finely fluidal arrangement of the material parallel to its contours, and 

 a crowded condition at its angles. Generally, however, this substance is isolated in 

 the midst of the brick-red felsitic material. In both cases, when both nicols are used 

 the fluidal structure disappears entirely and another crystalline structure stands 

 out. In ordinary light the distribution of the impurities when not adjacent to the 

 crystals gives the whole area the appearance of a summer cumulus cloud, the only 

 colors being different shades of a yellowish brown, passing to yellowish white; but 

 between crossed nicols a fibrous crystallization is apparent, standing radially perpen- 

 dicular to the exterior surface. This is sometimes in layers or aggregations which 

 correspond in general to the forms of the cumulus seen in ordinary light. These 

 crystalline fibres may be of thalite, since the hardness of these areas is so low 

 that they can be slightly crumbled into a floury powder under the thumb-nail and 

 have the opaque, sub-resinous lustre of that mineral. The openings in which these 

 crystalline fibres were thus arranged, somewhat in a botryoidal manner, were not 

 caused by shrinkage, like the cracks that exhibit the perlitic structure, but are an 

 incident of the flowage. The original substance was probably a form of the glassy 

 magma, slightly different from the rest of the magma, such that in being devitrified 

 it received the magnesian constituents of the glass and rejected the most of the iron 

 oxide. Although generally elongated they have not the origin, nor the axiolitic 

 structure described by Zirkel (Exploration of the Fortieth Parallel, vol. vi, page 166) 

 in the western rhyolytes. (See, also, under No. 571.) 



Three sections examined. 



Remarks. The above described perlitic structure is a well known character of 

 rhyolytes and glassy volcanic rocks. It is illustrated by the photograph seen in 

 figure 6, plate I. The central portion of this figure also contains a view of one of the 

 anorthoclases cut parallel to the brachypinacoLd (010) with a corroded embayment 

 in one corner. Figure 5, of plate I, shows the. same structure, here embracing a 

 quartz phenocryst with several embayments of the matrix. 



Mir. No. 140(8). Identical with No. 140(7), except that the coarse laminated 

 fluidal structure is wanting. The thin section, however, shows a micro-fluidal 

 structure, which arranges itself about the phenocrysts. 



Two sections. 



Mic. No. 140(9). Dark brown, with lighter laminations along the fluidal 

 partings, some of them being of quartz, some of the brick red substance composing 

 the bulk of No. 140(7) and some of them of a lighter red felsyte; contains quartz and 

 reddened feldspar phenocrysts. The sections show a minute fluidal structure. 



Two sections. 



No. 140(10). Similar to No. 140(9), but darker, yet spotted with a substance 

 that is nearly white like No. 140(12). These whitened spots are sometimes very 



