1922] Hudson: Geology of the Cuyamaca Region of California 185 



coarser than usual and carries knots of muscovite up to 4 mm. in diam- 

 eter. For a distance of twenty yards from the granitic rock the schist 

 is so permeated with igneous material as to form an injection gneiss. 

 The igneous rock here is a gneissoid granodiorite, composed of ortho- 

 clase, oligoclase, quartz, and biotite. The injection gneiss consists of 

 regular layers of pegmatitic material, from 1 mm. to 10 mm. in thick- 

 ness, separated by very thin layers of muscovite and biotite. The 

 pegmatitic layers carry tourmaline, quartz, and feldspar. There is a 

 gradual transition from schist carrying little or no injected matter 

 to gneissoid granodiorite, carrying a few shreds of what appears to 

 be schist. Contacts of this nature are to be found at almost all places 

 where granitic rock meets schist. 



The presence of tourmaline in these rocks is, however, not the 

 general rule. As an example of the more common type, the mineral 

 composition of an injection gneiss from the contact immediately west 

 of Julian may be cited. The rock here is composed of fine-grained 

 layers of albite, quartz, muscovite, and biotite, alternating with coarser 

 layers of quartz, muscovite, albite, and andesine. 



Many of the injection gneisses, especially those found along the 

 southwest contact of the main schist body, carry blunt lenses of quartz- 

 ose rock, like those found within the sillimanite gneisses. These lenses 

 have the appearance of fragments of once continuous layers that have 

 been rounded by rolling or kneading. 



Some of these lenses are moderately fine-grained paragneiss having 

 the appearance of hornfels. They invariably contain considerable 

 quartz and generally carry either pyroxene or basic plagioclase or 

 both. Biotite is generally lacking or is in very small amount. Those 

 specimens carrying the most biotite are the only ones having any 

 suggestion of schistose structure, but there is sometimes a layering 

 either on a megascopic or a microscopic scale, due to the distribution 

 and varying grain of the minerals. 



Other lenses consist of rock carrying from 75 to 80 per cent of 

 quartz, 15 per cent or more of pyroxene, the balance consisting of 

 basic plagioclase (basic labradorite to bytownite), biotite, graphite, 

 magnetite, titanite, apatite, and rutile. The pyroxene in three of the 

 specimens was diopside, while in another sample it proved to be augite. 

 The rock has a mosaic texture. The pyroxene often shows sieve tex- 

 tures. The crystalloblastic order is (1) biotite, (2) pyroxene, (3) 

 quartz and plagioclase. 



