36 THE OPEN BOOK OF NATURE 
secured the preservation of minerals which have 
enriched mankind. The great Scottish boundary 
fault is a case in point. It has turned up the 
edges of strata that would ordinarily have been 
buried deep below the surface, but which can 
now be walked over and examined; it has also 
preserved much coal, iron, and oil shale in the 
depression it has caused, which might otherwise 
have been broken up by the weathering agents, and 
have been washed away. 
Earth movements have actually thrust some older 
rocks over newer ones, and they have caused other 
rocks to be folded and crumpled. Folding is due to 
what is called “‘lateral pressure.”’ If you take a 
good-sized book, and press its edges between your 
hands, it will assume a wavy and crumpled appear- 
ance. Well, in some instances, as in parts of the 
Scottish Highlands, rocks have been subjected to 
such lateral pressure, and in consequence they are 
now crumpled and folded, as well as altered. I give 
you a picture of such a rock on Plate 12, c, and also 
in the accompanying illustration, Fig. 5. 
The inclination of rocks is called their dip, and 
when earth movements have caused the edges of 
rocks to appear at the surface, those edges are known 
as outcrops. | 
When you are rambling over the country with 
eyes open for interesting things, you are almost sure 
to find some evidences of the work of glaciers, which 
at one time in geological history were exceedingly 
extensive in Northern Europe and North America. 
During what is known as the Great Ice Age, Northern 
Europe was literally buried under vast ice-sheets and 
