FELDSPATHIC MAGMA. 255 



Feidspathic Magma. Up to this point the composition of the rocks has 

 been but little considered except as regards the mineral constituents of inde- 

 pendent lava flows ; it is necessary now, however, to look at them from the 

 standpoint of a series of successive eruptions in order to understand their 

 interdependence and geological relations. Normal hornblende-andesite, the 

 earliest and most basic portion of the feldspathic magma, passes over with- 

 out any recognizable physical break into hornblende-mica-andesite by the 

 coming in of hexagonal plates of biotite which gradually increase in amount 

 until they become the most prominent of the ferro-magnesian minerals and 

 at the same time by insensible gradations the hornblendes decrease. Grad- 

 ually the lava grows more and more acidic and quartz grains are developed 

 in the groundmass, but at first not in sufficient force to be regarded as an 

 essential constituent. The presence or absence of quartz is also governed 

 in great measure by the degree of crystallization of the magma, a highly 

 crystalline structure carrying more individual secretions than one where 

 silica is largely absorbed in glass. With the increase of quartz the horn- 

 blende continues to diminish and the rock passes over into dacite, the biotite 

 apparently holding its position with an occasional hornblende. 



In dacite, quartz has become an essential mineral. With a still larger 

 increase of the silica percentage orthoclase appears in broad and well devel- 

 oped crystals. Hornblende disappears entirely and in the normal varieties 

 of rhyolite the biotite is rarely seen and then only as an accessory min- 

 eral ; the feiTO-magnesian minerals are wanting. At the basic end of the 

 feldspathic series of lavas, labradorite and anorthite have been determined 

 by their optical pi-operties, but the predominating feldspars are apparently 

 oligoclase. By insensible gradation the lime-soda feldspars pass away. 

 Orthoclase, in most of the basic rocks, is entirely wanting, making its 

 appearance by degrees until at the acid end of the series it occurs as the 

 prevailing feldspar, although some species of plagioclase is nearly always 

 present. At one end of this series of eruptive material the essential min- 

 erals are hornblende and one or more species of lime-soda feldspars; at the 

 other, quartz and orthoclase. 



Pyroxenic Magma. The basic or pyroxene lavas began by the pouring out 

 of large masses of the Richmond Mountain pyroxene-andesite. Successive 



