12 INTRODUCTION. 



feature is a sure indication of a lapse of time. In other in- 

 stances we find certain strata overlapping the margins of 

 deposits previously formed, so as entirely to conceal them 

 over large tracts. Thus there is much uncertainty in the 

 distribution of the strata, but their order is never inverted 

 except in some rare cases of disturbance, and then, as a rule, 

 only over a small area. The older rocks are often at the 

 surface where the newer have either never been deposited 

 upon them, or have been subsequently denuded. Conse- 

 quently in coal-mining, it is always easy for a geologist to say 

 that coal cannot be found where the rock at the surface is 

 older than the Coal-measures, but it requires much considera- 

 tion to say where it might be found, and if so at what depth, 

 when newer rocks are at the surface. (See Figs. 23 and 24.) 



When we come to take the estimated thickness of the 

 known strata, and this is from 80,000 to 100,000 feet, and 

 compare it with what we know of the earth's crust through 

 the deepest boring, which is little over one mile in extent,^ we 

 at once discern what changes, denudations, and dislocations 

 the crust has undergone, by means of which the older rocks 

 are brought to the surface. For it may be safely assumed 

 that all the beds were originally deposited in an approxi- 

 mately horizontal position, and that any considerable devi- 

 ation from this, such as is often exhibited by the planes of 

 bedding, is due to subsequent disturbance. 



The study of the formation, alteration, and disturbances of the 

 rocks is termed Dynamical Geology. 



The ' lie,' or general inclination of the strata is called the Dip. 



The Strike is the line of outcrop of beds along a level surface ; it 

 is the line of greatest upheaval. In England the Secondary and 

 Tertiary beds dip generally to the south-east, consequently the 

 strike is north-east and south-west. In tracing the boundary-line 

 of a formation belonging to a conformable series, as the Carboni- 

 ferous or Secondary strata, the dip is a constant guide. Where 

 this takes a low angle of 2° or 3°, a very small irregularity in the 

 ground, a gentle hollow, may cause the boundary between two 

 divisions to run a long way from the line of strike ; whereas, with a 

 high dip, the lower beds would extend but a short distance even in 

 a deep valley or ravine. The edges of a formation, exposed by 

 denudation, are termed its outcrop or basset. 



While observing the general direction or dip of the beds, care 



^ Boring at Sperenberg, near Berlin, 4172 feet ; at Potsdam, Missouri, 5500 feet, 

 Colliery Guardian, March 10, 1S76 ; at Homewood, Pennsylvania, 6000 feet! 

 AthettcEtim, Nov. 21, 1885. 



