282 



UNSTRATIFIED ROCKS 



In a limited exposure it is often difficult to distinguish at once 

 between an intrusive and a contemporaneous sheet, but there are 

 certain characteristic marks which enable the observer to decide. 

 The presence of scoriae shows that the sheet is contemporaneous. 

 If, on the other hand, the overlying stratum be baked and altered 

 by the heat, or if the sheet cuts across the bedding planes at any 



Fig. 126. — Contact of intrusive sheet of diabase with shales. Base of Palisades, 

 Wiehawken, N.J. (U. S. G. S.) 



point, or if it can be traced to a dike which rises above it, or if it 

 gives off tongues or veins, or if pieces of the overlying stratum 

 be torn off and included in the sheet, it must be intrusive. The 

 nature of the contact between the sheet and the stratum above it 

 is also significant ; if the former be contemporaneous, the cracks 

 and fissures of its upper surface will be filled with the sediment- 

 ary material. Finally, the texture of the igneous mass gives valu- 

 able evidence ; in the intrusive sheet the texture is compact 



