224 WILLIAM H. EMMONS 



Under the microscope the following minerals are present in the order 

 of their abundance : alkali feldspar, quartz, andesine, magnetite, 

 hornblende, and biotite. The alkali feldspar is orthoclase, micro- 

 perthitically intergrown with albite, and occurs as anhedra which 

 reach a maximum diameter of 2 . 4°^"^. Quartz occurs as irregular 

 anhedra of equal size interlocking with orthoclase and sometimes as a 

 graphic intergrowth. A small amount of andesine (Abj AnJ is 

 present, as slender laths about 0.5"^™ long. These laths are con- 

 centrated along the margin of the vein and in many cases their long 

 axes are normal to the contact. Anhedra of magnetite as large as 

 Q_ ^mm jj^ diameter, fibrous green hornblende, and biotite are included 

 in orthoclase and quartz. Biotite is slightly altered to chlorite. 



The contact of intruding and intruded rock is sharp in the hand 

 specimen, but under the microscope these two rocks appear to grade 

 into each other within a zone about 0.2™™ wide. In this zone 

 some of the quartz of the intruding rock appears to have attached 

 itself with similar optical orientation to quartz in the intruded rock. 

 In the crystallization of the veins magnetite, hornblende, biotite, and 

 plagioclase were formed before alkali feldspar and quartz, and the 

 latter two crystallized synchronously. Perhaps all of the minerals, 

 except alkali feldspar and quartz had formed before the intrusion, 

 but if so the magma must have been very fluid, since these minerals 

 do not show flow structure and since the long axes of the plagioclase 

 laths, more frequently than otherwise, make a large angle with the 

 walls. 



The composition of the granite veins is not far from that of the 

 micrographic intergrowth of quartz and orthoclase which solidified 

 last in nearly all of the Haystack rocks. The veins were probably 

 formed during the latter stages of the solidification of the rocks they 

 cut, and may represent the liquid portion of the magma after most of 

 the plagioclase and nearly all of the ferromagnesian minerals had 

 crystallized out. Even the smallest veins are very coarse grained and 

 must have cooled slowly. This suggests that they were intruded 

 while the rock which they cut was still hot, or that their water content 

 was sufficiently high to permit the growth of large crystals. The 

 cracks they fill are probably shrinkage cracks formed directly after 

 solidification of the rock. 



