180 H. p. CUSHINCi Ar(,ilTK-SYEi\lTJi: GNEISS NEAJl J.OON LAKE 



same, and that a])pears like a somewhat crushed intrusive rock. When 

 it is recalled that the whole region has suffered profound dynamic met- 

 amorphism, producing a common foliation in the rocks irrespective of 

 their origin; when it is furtlier borne in mind that in the eastern half 

 of the Adirondacks the rocks of the Grenville (crystalline limestone) 

 series seldom occur in considerable belts, but rather in mere i3atches, 

 and these patches are often Avholly surrounded by undoubted igneous 

 rocks, being found not infrequently inclosed in the anorthosites, and, 

 further, that in the majorit}^ if not in all cases, the bedding of the one 

 and the foliation of the other are parallel, the seeming difficulty in the 

 interpretation disappears ; when, finall}^ the chemical nature of the 

 rock is taken into consideration the case for its igneous origin seems 

 made out. 



It is not certain from the section whether the clastic rocks are in place 

 and cut by large dikes of the intrusive, or whether they represent frag- 

 ments caught up by the molten flood. Many examples of the latter 

 might be cited from the eastern Adirondacks, and the great extent of 

 the syenite in the vicinity of Loon lake with the scarcity of the Gren- 

 ville series makes it the probable explanation here. Professor Kemp 

 concurs in the view that these patches represent the remains of a once 

 extended formation completely broken up by the great intrusions. 



Microscopic Character and Mineral Constituents 



The thin-section from the hand specimen chosen as typical and used 

 for the chemical anal3^sis shows the presence of the following minerals: 

 Zircon, apatite, magnetite, garnet, hypersthene, augite, hornblende, oligo- 

 clase, microperthite, and quartz. Other slides from the vicinity of Loon 

 lake show in addition biotite, titanite, allanite, and p3a'ite. The rock is 

 essentially composed of microperthite, augite, and hypersthene, with 

 (|uartz always present in varying and commonl}^ slight amount. 



Zircon and apatite are ver}' sparingl}'^ present, are the onl}^ constit- 

 uents with idiomorphic boundaries, and occur in the usual microscopic 

 crj'stals as the earliest crystallization from the magma. 



Magnetite is onh' in slight amount in irregular grains. 



Allanite and titanite are found in only one of the slides and in onh' 

 minute quantity. The titanite is of deep orange-brown color, like that 

 in the pyroxene-granulites found associated with the magnetites of the 

 Adirondacks. But two small fragments of allanite occur, so that its 

 diagnosis is perhaps not beyond question, though the optical projierties 

 agree wholly with those of that mineral. 



