GEOLOGY OF THE PARADOX LAKE QUADRANGLE 501 



there is considerable confusion, gabbroic . bands alternating with 

 anorthosite in an astonishing manner. The southern and eastern 

 boundary of the area was difficult to determine because of this alter- 

 nation and gradation of types. In the extreme eastern portion of 

 the area mapped as gabbro' there is a rock which seems to repre- 

 sent crushed gabbro. It consists of garnet, of almost, microscopic 

 size, which gives the rock in the hand specimen the appearance of 

 a granular aggregate of little garnets. The same rock is found in a 

 series of dikes on top of the mountain 1742 feet high, slightly north 

 of east of Peaked hill. 



In thin section the garnet rock is found to be a true gabbro, con- 

 taining green pyroxene, labradorite, diallage,.titaniferous magnetite 

 and garnet. These small dikes differ from the commoner bosses in 

 being of finer grain; in having relatively greater abundance of 

 garnets, and in the presence of diallage. 



The occurrence of the small garnetiferous dikes in the anorthosite, 

 and also along the contacts of gabbro and anorthosite, suggests 

 their peculiar structure as due to contact effects. They are certainly 

 a part of the gabbro intrusion, and their occurrence indicates that 

 the gabbro is later than the anorthosite. 



The gabbros are usually crushed, and then develop gneisses which 

 can not be distinguished from the gneissic development of the gabbro 

 phase of the anorthosite, nor from the gabbroic part of the syenite, 

 nor from some areas found among the granites. 



The gabbros are frequently granulated and show gradations sim- 

 ilar to those seen in the anorthosite. There appears also to have 

 been recrystallization in the gabbros. The plagioclase contains many 

 fine black inclusions which may be either pyroxene or titaniferous 

 magnetite, or both. They are apparently inclusions, not alteration 

 products. Similar inclusions have been described by many writers 

 on gabbros.^ Reaction rims are common. 



'G. H. Williams. U. S. G. S. Bui. 28. 



F. D. Adams. Uber das Noriaii oder Ober-Laurentian von Canada. Neues 

 Jahrbuch. Band 8, p. 425. 



A. C. Lawson. Anorthosites of the Minn. Coast of Lake Superior, Minn. 

 Geol. Survey Bui. 8. 1893. p. 8. 



J. F. Kemp. Gabbros on the Western Shore of Lake Champlain. Bui. 

 Geol. Soc. Am. 5:213-24. 



