GEOLOGY OF THE HAYSTACK STOCK 209 



The outlines of the sections of feldspar aggregates are for the most 

 part irregular but some are those of feldspar crystals. They vary 

 from 0.3™"^ to 1.5"^"^ in length. The matrix of the feldspar 

 aggregates is mainly orthoclase and included within it are minute 

 bodies of plagioclase, some approaching idiomorphism and showing 

 both carlsbad and albite twinning. They are from o.i to 0.2™"^ 

 long, and are oligoclase-andesine. They penetrate one another 

 most complexly. The plagioclase crystals of the aggregates do not 

 lie entirely within the outlines of the orthoclase, but along its edges 

 they penetrate the groundmass of the rock. The amount of plagio- 

 clase in the aggregates varies from 10 per cent, to 90 per cent, of the 

 whole, and the quantity of orthoclase is reciprocal with that of the 

 plagioclase. The other aggregates, composed mainly of ferromagne- 

 sian minerals, in some cases have outlines which strongly suggest the 

 crystal form of hornblende. These aggregates are composed of 

 mica, pyroxene, magnetite, feldspar, quartz, and apatite, but the 

 proportion of these constituents varies greatly in different cases. 

 In most sections of these aggregates, cut parallel to the c axis of 

 hornblende, the minerals are arranged in rows, while in oblique 

 sections and in those cut across the prism, the arrangement, some- 

 times roughly concentric, is usually quite irregular. The description 

 of a typical section parallel to the c axis follows. It is o . 9™"^ long 

 and 0.4"^"^ wide, and is composed of mica, magnetite, feldspar, 

 p)T:oxene, and quartz; mica and magnetite being most abundant. 

 There are four belts parallel to c each about o.i™'" wide; two of 

 these are composed chiefly of mica, while the alternate two are chiefly 

 magnetite. All the minerals within the aggregate are of small size, 

 though the mica flakes are larger than the anhedra of other minerals. 

 They are in various optical orientations ; a little quartz and feldspar 

 are present in all of the belts. Rimming these belts of mica and 

 magnetite is a colorless border consisting of very minute anhedra of 

 feldspar. It is o . 01 2 ^^ wide, and has the outline of a previous crystal. 

 Beyond this is a fringe of minute pyroxene crystals which approach 

 idiomorphism, and interspersed with these there is a small quantity 

 of allotriomorphic feldspar. The inner outline of this fringe is very 

 definite and sharp but the outer border is very irregular. Many of 

 the crystals of pyroxene join the border of feldspar at right angles. 



