CONSTRUCTION OF THE EARTH. 



120 



water. Each film or lamina was the result, of course, of 

 a separate deposition that is, after each lamina was 

 completed, there was a pause for a time in the deposi- 

 tion from the water. In those cases in which there are 

 fifty, or even a hundred laminae in the thickness of an 

 inch, the process, including the intervals, must have been 

 very slow. Some years must have been required to lay 

 down a foot of such a bed. Where the lamination is not 

 so fine, the accumulation of the sediment of which the 

 rock was made was more rapid. The term formation is 

 applied to a series of strata that have a relation to each 

 other in similarity of fossils, and which are therefore in- 

 cluded in the same age or period. This is the general 

 idea, but the word is used rather loosely, being made to 

 refer to a larger range of strata at one time than at an 

 other. In looking at a stratified rock, you observe be- 

 side the horizontal lines which indicate the divisions be- 

 tween the layers certain lines which cross these. The 

 two sets of lines are seen in Fig. 50. The lines which 



Fig. 50. 



run up and down across the layers indicate planes of di- 

 vision, which are called joints. Some of these, you see, 

 extend farther than others, and these are called master- 

 joints. There are commonly two sets of joints at rigj^t 

 angles to each other, as is represented in the figure. It 

 F2 



