132 NEW YORK STATE MUSEUM 



are, but it has also,, doubtless, been made equally clear that a 

 more definite meaning can be attached to them only as the result 

 of long and careful work. 



As yet, not one of these formations has been accurately de- 

 limited over any wide area, in part on account of the perplexing 

 intermediate varieties which are so frequent, in part on account 

 of the absence of good base maps, but even more because the work 

 has necessarily been done very rapidly. 



Under the most favorable conditions, the accurate mapping of 

 the region must be attended with much difficulty; and the results 

 as yet obtained must be regarded as far from final. 



This is particularly true of the gneisses, which are very complex 

 and troublesome. Some gneisses are igneous and some sediment- 

 ary in origin, without question, and it is probable that the former 

 class includes most of the gneisses of the region; but, at present, 

 it does not seem advisable to attempt any separation of the 

 gneisses, except to include in the limestone formation such as 

 appear certainly to be a constituent part of it. The gneiss, there- 

 fore, as represented on the map, must be regarded as by no means 

 a unit, though it is probable that a large proportion of it consti- 

 tutes a single formation, or at least is made up of rocks so closely 

 related in origin and age that they can not be separated. But 

 it surely comprises portions of the limestone formation, as well as 

 more or less modified later intrusive rocks, impossible to separate 

 except by close work. 



The limestone formation is indicated only where the actual 

 limestone is present or some of the characteristic associated 

 schists appear, and therefore there can be no question that many 

 small areas of the formation are not represented'. 



The same may be said of gabbro, various basic gneisses which 

 may possibly be modified phases of this rock being included with 

 the gneiss. 



Granite and syenite are indicated only when the proximity of 

 limestone makes easy the determination of their intrusive nature. 



Between the granites and syenites, on the one hand, and the 

 gneisses on the other, it is impossible to draw any sharp distinc- 



