STRUCTURAL TYPES OF PORPHYRITE. 



337 



scopical effect of this structure is to reuder the groundmass much less distinct in con- 

 trust with the crystals than is the case in the types of the main group. Flakes of 

 biotite, grains of magnetite, &c., are scattered about in this groundmass with the 

 same irregularity as in any other. 



A tendency to a micrographic-granite structure was noticed in two of the porphy- 

 rites. It seems to have been induced by the presence of the rounded quartz grains 

 above described. Each of these is surrounded by a zone in which quartz and 

 orthoclase are more or less regularly iutergrown. The appearance, as seen' under the 

 microscope, is that of alternate fibers of quartz and orthoclase, with a more or less 

 distinct radiate arrangement about the large quartz grain, all the quartz substance, 

 in both granule and groundmass, having the same optical orientation. A similar phe- 

 nomenon was not observed in connection with large particles of feldspars, and those 

 portions of the groundmass showing a regular intergrowth apparently independent 

 of any crystal may have been formerly related to a quartz grain situated just above 

 or just below the plane of the present section. 



In such a thoroughly crystalline rock a fluidal structure can only be expressed 

 by the position of the hornblende needles or biotite leaves with reference to the large 

 crystals. Such a relation is often observed, and it is also not rare to find hornblende 

 crystals broken and biotite leaves folded and crumpled, attesting to movements in the 

 partially solid rock. 



Structural forms The greater number of the rocks observed form a continuous 

 series whose extremes are very dissimilar, and the relationship can be most easily 

 understood and explained with the help of the subjoined table : 



Under Division A are included rocks with a light, homogeneous-appearing 

 groundmass, containing no microscopic individuals of the basic mineral which is so 

 prominent in macroscopic crystals, this in the one case (I) being biotite, in the other 

 (II) hornblende. Under 0, at the opposite structural extreme, where the groundmass 

 is filled with minute flakes or needles of a dark mineral, are also two modifications, 

 one a biotite (VI) (Fig. 1, Plate XX), the other prevailingly a hornblende rock (Fig. 

 2, Plate XX). These are both dark and compact, showing comparatively few macro- 



MON XII 22 



