INTRUSIVE SHEETS 



281 



north to south, and its thickness varies from 300 to 850 feet ; the 

 dike which supplied this immense mass is exposed in a few places 

 along its western edge. 



Intrusive sheets are most commonly formed in horizontal strata, 

 which offer less resistance to horizontal expansion than do the 

 folded beds ; they are also very generally of the most fusible 

 family, the basaltic, because such magmas retain their fluidity and 



Fig. 125. — Palisades of the Hudson, New Jersey. (Photograph by Rau.) 



flow for longer distances than do the highly siliceous rocks. It is 

 probable that intrusive sheets can be formed at only moderate 

 depths, because the overlying strata must be lifted to an amount 

 equal to the thickness of the sheet. At great depths the weight 

 to be lifted is so enormous, that the easiest path of escape must 

 be by breaking through and across the strata. If the beds are 

 subjected to compression after the intrusion of the igneous 

 masses, the latter will be flexed or faulted like the stratified 

 rocks. 



