GEOLOGY OF THE LONG LAKE QUADRANGLE 483 



their presence does not aid in the general solution of the problem 

 as to the age of the bulk of this gneiss, since if it be older it would 

 be apt to be cut, here and there, by outlying masses of the later 

 eruptives. Such masses are frequc ntl}^ found in it, but in general 

 the evidence does not permit th3 determination of their char- 

 acter, whether they cut the gneiss or belong with it. 



Gabbro. The gabbros are dar'c colored, basic rocks, usually 

 showing a reddish tinge owing to tlie presence of garnet. As found 

 within the quadrangle the rock occurs mainly in the dike form, 

 and these dikes have been noted cutting all the other eruptives, 

 with the exception of the granites, leaAdng the relative ages of 

 the two somewhat in doubt though the granite is thought to be 

 the older. In addition to the dikes is the small boss which cuts 

 the anorthosite south of Panther pond. The rock for the 

 most part is tough and resistant, and generally rather evenly 

 granular. It lacks the gneissoid character of the gabbro border of 

 the anorthosite, and weathers much less readily than that. It has 

 not been noted grading into ami^hibolite, after the fashion of 

 the gabbro found with the Long like gneiss. The smaller masses 

 and the dikes of that rock are always found in the amphibolite 

 condition, and this more metamorphosed condition seems to argue 

 a greater age, though it is possible to explain it as due to local 

 causes. The unchanged cores of that gabbro have also a more 

 pronounced ophitic structure than has been noted in this later 

 gabbro, though that also tends toward the same structure in the 

 larger masses. It is only with the greatest difficulty that the 

 rock can be distinguished from that of the dikes of gabbroic syenite. 

 This will be later reverted to. 



As a possible exception to the above statement the gabbro knob 

 at the farm in Litchfield park, just west of Jenkins pond, must be 

 instanced. This is thoroughh metamorphosed to a micaceous 

 amphibolite, yet is an unquestioned gabbro; nevertheless it is 

 entirely surrounded by granitic syenites, regarded as belonging to 

 the later eruptives. Unfortunately no contacts show and the rela- 

 tions between the two rocks can not therefore be made out. It 

 would seem to be easiest acc('unted for on the assumption of a knob 

 of later gabbro cutting the syenite. It is rather large for an in- 

 closure in the syenite, yet the writer's present disposition is to re- 

 gard it as such, since the Long Lake gneiss is close at hand to the 

 south. If it be not, it of ..;ourse vitiates the attempt made above to 



