September, 1911. 



KNOWLEDGE. 



337 



great faults between niucli more ancient rocks. In 

 the deep fissures and cracks produced by this 

 motion huge floods of basaltic lava have welled up 

 while the sinking blocks of the crust sank into their 

 place. The type of structure thus produced is 



shown in Figure 1. .. _ 



The mountain-building /' 



movements are probably ; 



caused by the gradual 

 contraction of the earth 

 as it cools in the long 

 course of ages. The 

 molten interior contracts 

 faster than the solid crust, 

 which is kept at a more 

 constant temperature on 

 account of radiation, 

 atmospheric circulation, 

 and the heat received 



from the sun. From time to time, to be strictK' 

 accurate, at all times, the outer crust is being 

 drawn inwards by gravit\'. One effect of this ten- 

 dency has already been noticed, viz., the vertical 



Figure 5 



movement is to originate a series of folds such 

 as are shown in Figure 2, dipping in the direc- 

 tion of the movement and striking across the 

 countrv, in a direction at right angles to it. As the 

 mo\ement intensifies the limbs of the folds become 



steeper and steeper, the 

 more brittle strata snap at 

 the crests of the folds as 

 in the great anticlinal fault 

 which marks the Pennine 

 Chain in Lancashire and 

 Yorkshire (see Figure 5). 

 At last the folds them- 

 seh-es become folded, 

 that is to say, their axes 

 bend o\-er and isoclinal 

 folds are produced in 

 which both limibs dip in 

 the same direction, and 

 the whole system of folds and faults is arched 

 and crumpled up. as shown in Figure 4. In 

 this intense crushing the limbs of folds are often 

 so compressed as to squeeze the core of the fold 



tU^.sb 



Figure 6. 



slijiping downwards of some areas of the earth's 



crust relativeh' to others, or plateau formation. But 



since a small sphere has necessarilv a smaller area 



than a larger one, it follows that the bodil\- sinking 



of one portion of 



the earth's crust Cambrian itra^a 



squeezes together "-""'^'s" 



other parts, and 



this movement or 



pushing aside of 



parts of the crust 



is horizontal or 



tangential. The 



pressure of this 



movement is so 



enormous and irresistible that the solid rocks of the 



crust are folded, crumpled, and piled on top of each 



other in their endeavour to occupy a less horizontal 



space. It is to the piled-up masses of strata raised 



in this way that we properly give the names of 



mountains, and the detailed stud\' of the way in 



which their component beds have moved is of 



peculiar interest. 



The effects produced vary according to the nature 

 of the strata. Igneous rocks and quartzites are 

 very brittle and have a great tendency to break 

 when folded, but soft shales can be very highly 

 crumpled without fractures. \\'ith the less brittle 

 rocks the first effect of a horizontal earth 



I'IGURE 7. 



bodil\- into the crest gi\ing rise to fan structure 

 (see Figure 5), and the component grains of the 

 rocks themselves are broken and drawn into 

 parallelism with the strike of the folds. 



The extreme 

 brittleness of 

 many rocks gives 

 rise to another set 

 of features, and 

 since both brittle 

 and more or less 

 flexible rocks are 

 usually found 

 close together 

 these effects are 

 the recent re- 

 been shewn that 



usually added. According to 

 searches of Mr. Cadell. it has 

 when horizontal pressure is applied to a brittle mass, 

 the parts nearest the pressure instead of folding, 

 break off and slide upwards on somewhat inclined 

 planes. .\s the block slides up the pressure is 

 brought to bear on the next portion of the mass and 

 another fracture or, as it is termed, minor thrust, is 

 produced. In this wa}- a series of thrusts is formed 

 dipping towards the applied pressure. At last the 

 huge mass of strata piled up between the pressure 

 and the undisturbed rocks is dri\'en forward bodily 

 along a genth'-inclined plane till it rests on undis- 

 turbed rocks. The part overlying this plane often 



