400 GEOLOGY 



no internal lamination or stratification, and the resulting rock is 

 usually classified as massive rather than stratified; but when a 

 succession of flows occurs, each individual flow forms a layer, and 

 the series as a whole becomes stratiform. The successive flows are 

 not usually co-extensive. If the later flows of the closing stages 

 of a period of vulcanism fail to reach as far as the earlier ones, a 

 terrace or step-like aspect is given to the region, whence the name 

 trap^rock ((trappe, steps) is derived. Such lava sheets, especially 

 if of basalt, often assume a columnar structure in cooling, the 

 columns being rude six-sided prisms standing at right angles to 

 the cooling surfaces (Figs. 8 and 308). This phenomenon is some- 

 times well developed in sills (p. 15). The formation of the col- 

 umns is sometimes regarded as a variety of concretionary action, 

 but more commonly as a result of contraction (p. 17). The 

 various structural forms due to intrusions of lava have been enu- 

 merated (p. 367). 



Metamorphic Rocks 



The conspicuous structural feature of metamorphic rocks is 

 their schistosity (p. 14). Crumpling has followed the develop- 

 ment of the schistose structure in many cases. Slaty cleavage, 

 which represents incipient metamorphism, has been referred to 

 (p. 53), as have the causes which produce metamorphism. 



Features Arising from Disturbance 



Inclination and folding of strata. The original attitude of beds, 

 whether formed by water or by lava-flows, commonly departs but 

 little from horizontality. Both kinds of deposits, however, occa- 

 sionally take place on considerable slopes. Modifications of the 

 'original attitude result from earth movements, and the measure- 

 ment of these modifications is an important feature of field study. 

 The position of beds is recorded in terms of dip and strike. The 

 dip is the inclination of the beds referred to a horizontal plane, 

 as illustrated in Fig. 309, and is usually measured by a clinometer, 

 shown in Fig. 310. In measuring the dip, the maximum angle is 

 always taken. The direction as well as the amount of the dip is 

 to be noted. Thus dip 40, S. 20 W., gives the full record of the 

 position of the bed of rock under consideration. The strike is 



