52 THE EABTH. 



had other strata deposited upon them, which again may be 

 more or less contorted from the same causes. The regularity 

 of strata is often interrupted by what are called " faults " 

 or " dykes," which have arisen from some part of the earth 

 sinking down or another part being raised up, producing a 

 fracture through all the strata and causing those on either 

 side of this fault or fracture to occupy a situation not 

 corresponding to those on the opposite, as in fig. 6, but yet 



PIG. 6. 



to be found at a higher or lower elevation, and it is nearly 

 always found that these strata are raised on that side to 

 which the line of fracture inclines, as in the figure. These 

 faults are often sources of great annoyance to the miner, 

 who finds a sudden termination to the seam of coal or ore 

 that he is working. The cracks are generally filled with 

 some primitive rock, as basalt, rising from beneath while in 

 a liquid state and filling up the interstices ; it will often 

 happen that the faults thus filled will have veins of the same 

 rock branching out and not only filling up cracks but forcing 

 its way between the various strata, thus interposing a 

 stratum of basalt quite out of its proper position and 

 altering, by the effects of the heat communicated, the 

 character of the strata in juxtaposition ; this is shown in 

 fig. 7. 



Upon a close examination of the various strata which form 

 the crust of the earth, it is found that each has its own 

 peculiar character ; some have resulted from the accumula- 

 tion of matters deposited at the bottom of ancient seas, 

 others in the beds of rivers or fresh-water lakes, or again 

 others (as the coal formation) from the accumulation of 

 vegetable matters ; further, these strata do not only differ 

 in "structure and composition, but also in the remains of 



