MACKIE ROCKS AND THETR TEACHINGS. 99 



thickness as the depression or elevation of the old continent on which 

 they rest went slowly on— thus : 



LiGN. 12. Ltgx. 13. 



When these sediments were elevated into dry land, and, as they rose out 

 of the waves, being subjected to the denudation or slicing-off of their 

 upper extremities, they would offer the conditions represented below — 

 thus : 



Ltgn. 14. — Strata thinning ont on an old uneven sea-bod. 



Suppose the beds to be of equal thickness ; then we should not always 

 get a true idea from their outcrops of the number of the beds 

 beneath the point a}\ (Lign. 16), and conse(juently we might be as far 



L 2 



