GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



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So far as the writer is aware, these are the only observations 

 so far put on record in the Adirondack region which have any 

 bearing on the relative age and relationships of the two rocks, 

 syenite and anorthosite. 1 They definitely show (1) that the 

 anorthosite is cut by a basic syenite, which is therefore younger; 

 (2) that this basic syenite shows considerable variation from 

 place to place and in one exposure bears a strong resemblance to 

 the normal syenite; (3) that at times the normal syenite shows 

 gradation into a gabbroid phase, best shown at Diana and Tup- 

 pep lake, which is similar to the rock which cuts the anortho- 

 site; and (4) that the anorthosite gabbro itself shows a tendency 

 to the production of a similar rock by local differentiation, that 

 is, a rock richer in pyroxene-hornblende-garnet content than the 

 ordinary anorthosite gabbro, but with oligoclase andesin in- 

 stead of labradorite for the feldspar and with the development 

 of much microperthite and some quartz in addition ; and that 

 at least one dike of a like rock occurs cutting anorthosite. But 

 it has not yet been demonstrated that the syenite found cutting 

 the anorthosite is connected with the main masses, yet such a 

 demonstration is necessary in order to definitely prove the 

 younger age of the latter. It may be argued however that dikes 

 from a syenite mass would draw their material from its 

 peripheral portions, and. if any differentiation had previously 

 taken place, would naturally be more basic than the main mass. 

 The Diana case proves that such differentiation has taken place 

 on a considerable scale; it has also taken place at Tupper lake, 

 and it is thought that plentiful evidence of the same sort would 

 be forthcoming elsewhere, were it not for the general unsatis- 

 factory character of the rock outcrops which prevail in the 

 woods. It is therefore thought that the evidence strongly 

 points to the occurrence of a considerable body of syenite in the 

 region which is younger than the anorthosite. The occurrence 

 of the syenite in a great number of separate masses renders it 

 possible how r ever that there may be some considerable age dif- 

 ferences between them. And the fact that in many parts of the 

 region there occur numerous small masses of similar rocks and 

 some larger ones too. which are thoroughly gneissoid and much 



iFor details see N. Y. State Geol., 20th An. Rep't. p.r35-r46. 



