10 



PRELIMINARY OUTLINE 



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

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

 rocks were formed first, and the lavas intruded later. Lava sheets 

 intruded between beds of stratified rock can be told from those 

 which flowed out on the surface and were subseauently buried, for 



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



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 

 metamorphic 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 

 the rock has undergone since it was formed. The structure shown 

 in Fig. 4 is known as schistosity a structure characteristic of 

 much metamorphic rock. Metamorphic rock may be derived from 

 both igneous and sedimentary rocks. 



More commonly than otherwise, metamorphic rocks lie beneath 



