GEOLOGY OF THE PARADOX LAKE QUADRANGLE 487 



therefore probable that most of the dikes belong with the gabbro. 

 Nevertheless some of the basic portions of the syenite mass present 

 the mineralogy of a gabbro, and if sheared would pass into 

 amphibolites. 



Summary of evidence of relative age of igneous rocks 



The anorthosite is cut by gabbro at Johnson pond and on Moose 

 mountain ; the syenite is cut by dikes of gabbro on Bull Rock moun- 

 tain (called Old Fort on the map), at Chilson, and at the foot of 

 Cat mountain. 



The relation between syenite and anorthosite is doubtful, no con- 

 tacts between the two having been found. Dikes, possibly of 

 syenite, cut anorthosite on Blue Ridge mountain. 



The relation between syenite and granite is also doubtful, the two 

 types often appearing to grade into each other. 



The granite is cut by dikes of amphibolite ; some of these appear 

 to belong with the gabbro; others are altogether doubtful and may 

 belong in age with the syenite. 



Gabbro is thus the youngest in age ; anorthosite, syenite and 

 granite are undoubtedly of nearly the same age and derived from the 

 same source since they present gradations towards each other. 

 Anorthosite, syenite, granite, is the most probable order of intrusion. 



Pegmatites. The origin of pegmatites has furnished occasion for 

 much discussion in the past. The close relations of pegmatites to 

 igneous rocks and their occurrence as dikes have led many observers 

 to regard them as true intrusives. On the other hand, their coarse 

 structure and frequent association with quartz veins have led others 

 to the reverse view, namely, that solution was too prominent in their 

 production to admit of an igneous origin ; that they are essentially 

 veins, and that they are genetically related to igneous rocks, with 

 more or less of pneumatolytic action at their time of consolidation. 



In the northeastern corner of the quadrangle, in the neighborhood 

 of Towmer pond, there is limestone in close proximity to intrusive 

 syenite and granite. Contact effects are to be seen in the presence 

 of enormous pegmatites. Roe's spar bed is a famous locality for 



