A. Wandke— Study of Cape Neddick Gabbro. 299 



Contact phase. — This phase forms a peripheral zone of 

 tough hard rock that rarely exceeds five feet in width. 

 Inclusions of quartzite in various stages of assimilation 

 are locally abundant. In the hand specimen the rock has 

 a dark gray color, is fine-grained although the crystals 

 of olivine that are abundantly sprinkled throughout the 

 specimen may attain a maximum diameter of three milli- 

 meters. Pyroxene and plagioclase can also readily be 

 identified. The presence of rather abundant apatite in 

 this contact phase evidently indicates that volatile com- 

 ponents played an active part in assimilating the engulfed 

 quartzite, the interstitial quartz and micropegmatite 

 being corroborative evidence of such action. Judged 

 from its mineral composition the rock is a hybrid and 

 resulted from the assimilation of quartzite by gabbro. 



Gabbro. — The gabbro forms an interesting phase of 

 this intrusive. In part it is developed as a normal 

 course-grained dark colored igneous rock, but for a width 

 of twenty-five to fifty feet from the contact phase it is 

 strikingly banded. These bands rarely over three inches 

 wide follow the contact, stand vertically and are due to 

 slight fluctuations in the relative amounts of feldspar and 

 ferromagnesian minerals. As distance away from the 

 contact is gained this banding becomes vague and disap- 

 pears, the rock then having the appearance of normal 

 coarse-grained gabbro. In the bands the order of crys- 

 tallization of the essential constituents seems to have 

 been olivine, magnetite, plagioclase, pyroxene, horn- 

 blende, and biotite. The olivine, never abundant, occurs 

 in grains with an anhedral outline that indicates resorp- 

 tion. The magnetite is irregular in outline, is commonly 

 enclosed in the feldspar sometimes occurring as dust-like 

 particles, and also occurs in the pyroxene surrounded 

 by a corona of biotite. Plagioclase feldspar (Ab 2 An 3 ), 

 the most abundant single constituent, was one of the first 

 minerals to form and its period of crystallization over- 

 lapped that of the pyroxene, hornblende and biotite. It 

 is frequently euhedral in outline, the ferromagnesian 

 minerals being moulded about the well formed crystals. 

 The pyroxene characteristically alters to hornblende and 

 biotite, a change that appears to have been intermittently 

 favored. It may well be that since they follow the pyrox- 

 ene both hornblende and biotite, minerals containing 

 hydrogen and artificially produced only in the wet way, 

 indicate the presence of mineralizers. 



