2o6 WILLIAM H. EMMONS 



by the serpentine, and the serpentine includes minute bodies of 

 magnetite. In all the rocks containing olivine a portion of it occurs 

 in small, rounded, oval, or irregular inclusions in the pyroxene, and 

 in specimens where only a small amount of olivine is present all is so 

 included. The quantity of olivine varies from nothing to 5 per cent. 

 Apatite occurs as slender prisms about .1™"^ in length, wholly 

 or partially included in the other minerals. In some specimens the 

 prisms are arranged roughly parallel to one another, somewhat 

 analogous to flow structure sometimes exhibited by lath-shaped 

 feldspar in basalts, though the alignment is less perfect. These 

 parallel prisms are included in plagioclase, orthoclase, and in the 

 ferromagnesian minerals, showing that the crystallization of the 

 apatite had begun but that the other minerals had not formed when 

 the magma came to rest. The amount of apatite is nearly uniform 

 and it usually constitutes about i per cent, of the rock. Pyrite occurs 

 sparingly except in altered varieties where it is in part secondary. 

 A very small amount of zircon is present in some specimens as char- 

 acteristic prisms. 



Description of Types of Haystack Stock 



VARIETY A. ADAMELLOSE: GRANODIORITE 



The most acid part of the stock covers a considerable area' on the 

 southern portion of the west spur of Baboon Mountain. It occurs near 

 but not immediately at the edge of the stock. The rock is light gray, 

 medium-grained, and has a smaller percentage of ferromagnesian 

 minerals than any other of the stock rocks, hornblende, magnetite, 

 and biotite constituting less than one-sixth of the mass. It is com- 

 paratively homogeneous in composition, although southward it 

 grades into a more basic and more coarsely crystalline diorite. 



The pinkish-white feldspar anhedra are from 3 to 5™°^ long and 

 the quartz is of smaller size. Hornblende is the most abundant ferro- 

 magnesian mineral, though magnetite and biotite are present. 



Under the microscope the texture of the rock is seen to be hypidio- 

 morphic granular. The minerals present in order of abundance are 

 plagioclase, orthoclase, quartz, hornblende, biotite, magnetite, and 

 apatite. Plagioclase occurs in nearly idiomorphic forms, tabular, 

 parallel to the brachypinacoid. Many carlsbad twins and zonally 



