58 



PHYSICAL GEOLOGY 



2. Which of the two dikes in Figure 39 is the older ? Reasons. 



3. What is the age of the lava sheet L (Fig. 40) in comparison 



with the age of the sedimentary 

 beds S and Si, (1) if the lava 

 sheet is intrusive, (2) if it is ex- 

 trusive ? 



4. What is the relative age 

 of the dike and the sedimen- 

 tary beds (S, Si, and 2 ) in 

 Figure 41 ? 

 FIG. 39. -Diagram of dikes. 5 Compare and contra st 



the texture of the rock in a thin and a very thick dike ; at the sur- 

 face and in the central portion of a massive lava flow. 



FIG. 40. Diagram of lava FIG. 41. Diagram of dike 



sheet between sedimentary and associated sedimentary 



beds. beds. 



6. Did the lava of the bombs shown in Figure 14 solidify before 

 or after striking the ground ? Reasons. 



7. Might the fact that 

 a given lava plateau had 

 been built up by several 

 distinct flows be told by the 

 texture of the rock ? If so, 

 how? 



8. How do igneous rocks 

 come to be at the surface? 



9. Coarse-grained gran- 

 ites, schists, and gneisses 

 outcrop at many points in 

 the uplands of southern 

 New England. Where were 

 these rocks formed with 

 reference to the surface ? 

 What inference, therefore, 

 may be made concerning the 



1 



4- 5MILE5 



amount of erosion which has 

 occurred in the region ? 

 10. (1) What is the rela- 



FIG. 42. Generalized map of small area 

 southeast of Port Orford, Ore. Short 

 lines represent igneous rocks ; horizontal 

 lines ssdimentary rocks, with some meta- 

 morphics. 



