204 IRVING. 



tively shallow seas, and no series of limestones exists between 

 it and the strata in question to indicate a gradual sinking. 

 Such a profound and sudden subsidence could hardly have oc- 

 curred without a disturbance essentially cataclysmic, and to the 

 writer's knowledge no evidence of such a disturbance is at hand. 



The writer would then incline to the first explanation and at- 

 tribute the absence of the Devonian partially to that explana- 

 tion, and partially to the belief that the missing series may be 

 represented by still unidentified strata. Furthermore, breaks 

 may exist, which are as yet undetected, because unconformities 

 which might be produced in shallow seas of this description are 

 not marked, and are difficult of recognition. Close study, such 

 as is necessary to establish their absence, has not yet been put 

 upon the subject. Investigations are now in progress by the 

 United States Geological Survey, and will probably throw light 

 on this much mooted question. 



Subsequent to the Cambrian, a gradual subsidence seems to 

 have occurred, during which the Carboniferous strata were laid 

 down, and following their deposition a gradual elevation began 

 which seems to be still in progress at the present day. 



IV. ERUPTIVE ROCKS. 



/. STRUCTURAL AND DYNAMIC RELATIONS OF 

 ERUPTIVE ROCKS. 



A. DESCRIPTIVE. 



' The eruptive rocks present in their structural relations three 

 easily separable and distinct groups, which are found respec- 

 tively in the three great stratigraphic formations of the hills. 

 They are therefore discussed under the following subdivisions : 



a. Intrusions in the Algonkian. 



b. Intrusions in the Cambro-Silurian. 



c. Intrusions in the Carboniferous. 



a. Intrusions in the Algonkian. 



Of the intrusions in the Algonkian formation we have but a 

 single type — dikes. These may be subdivided into two distinct 



