RELATIONS OF IGNEOUS ROCKS TO STRATIGRAPHY. 923 



When a series is cut by intrusives it sometimes happens that there is a 

 difference in the erodibility of the dike and of the containing rock. In case 

 the dike be more resistant, and the two were subjected to eroding agencies, 

 it protrudes from the underlying formation, while in the opposite case there 

 is a hollow. The latter is the condition of affairs described by Pumpelly 

 at Hoosac Mountain, where an ancient dike cuts through the gneissoid 

 granite. After a hollow of differential erosion was formed a sedimentary 

 series was deposited. In approaching the dike the members of the sedi- 

 ments thicken somewhat and pass into the depression. The greater thick- 

 ness of the lower beds at the hollow finally compensates for it, and the 

 thickened beds gradually pass up into ordinary regularly stratiform beds. 

 (Fig. 24.) In this case later orogenic movements produced consonant 

 secondary structures both in the granite and in the overlying formations, 

 and this relation of the dike to the two series was the first eAddence found 



c 



1 1 1- /i 

 "&. ■ • ' ' - 



Fig. 24.— Conglomerate deposited in depression produced by erosion of basic dike through gneiss. After Pumpelly. 

 C, conglomerate; c, lower layers of conglomerate rendered schistose by admixture of material from the altered dike; d, 

 diabase of the dike rendered schistose by metamorphism; e, altered dike material; <7, pre-Cambrian granitoid gneiss. 



of a structural break between them. They had hitherto been considered 

 as conformable, although a very close study had been made of them. 

 Later other evidence confirmed this inferred unconformity. 



In a different case the contacts between two unconformable series 

 may not be found, but one series may be cut through and through by dikes 

 and contain bosses of igneous material and interstratified intrusives or 

 extrusives, with perhaps also volcanic fragmentals, while the adjacent set of 

 formations may be wholly free from igneous rocks. Relations of this kind 

 have force in proportion to the extensiveness of the phenomena. If one 

 series is rich in igneous rocks for many miles, while another contiguous 

 series is entirely free from them, and the irregular distribution of the two is 

 such as not to be explained by faulting, the evidence of a structural break 

 between the series is very strong. 



In any of the above cases the time break between the two series must 

 have been long enough for a full cycle of igneous activity. 



