no NEW YORK STATE MUSEUM 



the northwest. It is clear in these cases that the Reservoir granite 

 cuts the gneiss and also includes many xenolithic blocks of it, and 

 it is clear also that these blocks had a complex structure before their 

 inclusion in the granite mass. 



It seems allowable, therefore, to postulate an igneous history of 

 some sort earlier than the great granite invasion and to credit at 

 least a part of the dioritic gneisses to this time. Acting on this 

 evidence, in the tabulation of formations and in giving the succession 

 of events, we have placed an igneous epoch with development of 

 bases injections between the time of Grenville regional metamor- 

 phism and the great granite invasions. 



Again at a later time, apparently after the great bathyb'thic mass 

 bad entirely ceased to function, there were igneous developments of 

 a quite independent nature. One of these was the Cortlandt series 

 of norites. gabbros and granites in the Pcekskill vicinity, which 

 probably belongs to a later period and not to the Precambrian at all. 

 As a matter of fact, however, the age of the Cortlandt series is not 

 known. 



There are, however, smaller units of Precambrian age repre- 

 sented by dioritic and basaltic dikes, perhaps best referred to as 

 basic dikes, which cut all of the formations up to the Cambrian. 

 They are particularly abundant in the Storm King granite and asso- 

 ciated gneisses. What relations these have to the larger series is 

 quite unknown. They are clearly Precambrian since none of them 

 cuts the Poughquag or Cambro-Ordovician series although they 

 occur abundantly in the vicinity of the remnants of these formations. 

 It is not beyond possibility, of course, that these dikes are repre- 

 sentatives of the final stages of the same great bathylithic mass, but 

 their strikingly different habit leaves that matter in complete uncer- 

 tainty and the best that can be said is that they are much later and 

 apparently independent. 



A very extreme type of basic rock was found at one point in the 

 quadrangle, essentially dunite. Only two specimens were gathered, 

 both from the same occurrence, and the relation to the country rock 

 is not very clear. Portions of the dunite are extremely sheared and 

 deformed but beyond that there is no help toward a correlation. 



There is undoubtedly still greater detail of Precambrian igneous 

 history than is indicated in the outline above. Units have been 

 seen in the field which are not readily matched with any of the 

 fundamental ones which form the basis of this statement, but it is 

 believed that they are in all cases incidental or minor developments 



