24 ANNALS NEW YORK ACADEMY OF SCIENCES 



The syenites are thus a fairly well distinguished group, although that 

 of the northern area is related to the gabbros and that of the interior is 

 almost a monzonite. It differs from them chiefly in the prominence of 

 the orthoclase and in the considerable amount of apatite, this mineral 

 being present in very small quantity in the typical Cortlandt monzonitic 

 diorites. There is little of interest about this area, but the location of 

 the other two between mica schist and biotite norite is very suggestive, 

 taking into account the similarity between these two widely separated 

 areas. A discussion of the possibilities of contact action must be reserved 

 for another portion of this paper, but it may be noted here that if it be 

 possible for the molten norite magma to act upon the mica schist to form 

 a rock of normal igneous composition, this third rock would probably be 

 approximately syenitic, both in chemistry and in mineralogy. No analysis 

 of the syenite was made; but the analyses of biotite norite and of mica 

 schist, given below, 23 may be compared. Williams 24 suggested the same 

 idea with reference to the gabbros, supposing that they always appeared 

 between limestone and norite. His premises are not altogether correct, 

 but the general idea can scarcely escape one who has worked on these 

 rocks. 



Sodalite Syenite 



Sodalite syenite was found in only one place, and then in a very small 

 place. It is interesting chiefly from a theoretical standpoint, as the only 

 representative in the series of the feldspathoid group. Its field relations 

 are somewhat peculiar. It is located at the north end of the road, imme- 

 diately east of Lake Meahach, on the contact with the inclusion of quartz 

 schist found at that place. Starting from the south, we have the sodalite 

 syenite (whose exact extent is indeterminable, but probably small), then 

 the schist, then a light colored augite monzonite in a strip about five feet 

 across, following by schist again. This is cut about 20 feet farther by a 

 much altered minette dike, three feet wide; beyond this the rock is 

 largely covered, and the exact contact of the schist on the north is hid- 

 den. It is thus impossible to say whether the sodalite syenite is a thin 

 contact strip surrounding the schist or not, although it is certain that it 

 does occur on the southern border. The significance of the augite mon- 

 zonite is difficult to perceive; it is not a dike and is therefore probably a 

 tongue of the country igneous rock projecting into the schist and partly 

 altered by it. 



M See pages 61 and 65. 



21 Amer. Jour. Sci., (3), XXXV, 440. 1S88. 



