REPORT OF THB DIRECTOR AND STATE GEOLOGIST 1900 r59 



Series 3 was constituted to include the rocks of the gabbro 

 group, both anorthosite and gabbro. Though, again, the subdi- 

 Tision is a lithologic one, Kemp held that these rocks were later 

 than those of both the other series, and the time element was 

 made of importance in the characterization of the group. 

 Further, at the time the classification was proposed these gabbro 

 rocks were the only ones known to be later than the rocks of 

 series 1 and 2 in the Adirondack pre-Oambrian. 



Thfs classification was proposed as a working hypothesis sim- 

 ply, and ha® been of the greatest service in all subsequent work. 



All the Adirondack pre-Potsdam rocks, excluding certain dikes 

 which are not metamorphosed, but which the writer has shown 

 to have preceded the Potsdam, are included in these three series. 

 Regarding the matter from the lithologic standpoint, the effect 

 has been that we have classified everything in series 1 which 

 €Ould not be unmistakably shown to be clastic, and hence to 

 belong to series 2, or to be a gabbroic rock and hence a member 

 of series 3. As a result, series 1 has come to include a great 

 variety of rocks, likely of very diverse age, and we all recognize 

 that the great problem of Adirondack pre-Cambrian geology is 

 the proper discrimination and separation of these diverse 

 elements. 



Since, however, the gabbro rocks are distinguished from those 

 of the remaining series, not only from the lithologic standpoint, 

 but also because they are clearly of eruptive origin and because 

 they are younger, it would seem philosophical to enlarge that 

 group in order that it may contain not alone gabbros, but also 

 all rocks which can be shown to be of igneous origin and to be 

 akin in age to the gabbros. This is in fact the position taken by 

 Kemp, the age difference being deemed of more importance than 

 the lithologic. 



In the writer's judgment a large (it is likely the larger) part of 

 the rocks which have heretofore been classified with series 1 can 

 be shown to be eruptives and to be in all probability as young as 

 the anorthosite. So probable does this seem that it is quite 

 justifiable to point out the effect from the classification stand- 



