14 GEOLOGY 



Fig. 3, where some of the igneous rock, indicated by black, is repre- 

 sented as lying beneath the stratified rock, and some above it, 

 while some is interbedded with the stratified rock, and some cuts 

 across its layers. From these relations it is possible to tell some- 

 thing of the order in which the rocks were formed. Where the 

 stratified rocks are broken through by lavas, it is clear that the 

 stratified rocks were formed first, and the lavas intruded later. 



Fig. 4. Metamorphic rock. (Ells. Can. Geol. Surv.) 



Lava sheets intruded between beds of stratified rock can be told 

 from those which flowed out on the surface and were subsequently 

 buried, for in the former case the sedimentary rocks, both above 

 and below the igneous rock, were affected by the heat of the lava, 

 while in the latter case only those below were so affected. 



Most metamorphic rock has cleavage, that is, a tendency to break 

 in one direction rather than in another. The cleavage of metamor- 

 phic rock may look much like stratification, but it is really very 

 different. The tendency to break along certain planes is not due 

 to the fact that the rock was deposited in layers originally, as in 

 the case of stratified rock, but is the result of the changes which 



