336 



THE NATURE BOOK 



ejectamenta and lavas of ancient vol- 

 canoes. At Castle Head, near Keswick, 

 the actual vent through which the lavas 

 were extruded stands like a monument 



to the long extinct fires. 



Where 



denudation has been active and long 



destruction has taken place. More fre- 

 quentlv, however, they are not so easily 

 destroyed, and remain as outstanding 

 masses when the country rock has been 

 removed. 

 Snowdon owes its pre-eminence to 



'SLATE HILLS HAVE A TENDENCY TO ASSUME 

 Swindale Beck and Dufton Pike. 



Photo.: i-ap/i h (''■ Bim^ley, lUadingley. 



A CONICAL FORM." 



continued, the very heart of the volcano 

 may be exposed. The Tors of Dartmoor, 

 Eskdale, Criffel and Peterhead are only 

 a few out of many which might be cited 

 where a once deep-seated liquid reservoir 

 has been frozen into solid granite and 

 then stripped of the superincumbent 

 strata. 



Besides the material thrown out over 

 the surface of the land or spread over 

 the sea floor, volcanic action results in 

 the thrusting of sheets and tongues of 

 liquid rock through the strata, and these, 

 in cooling, become sills and dykes. 



Some dykes are more susceptible to 

 decay than the surrounding rocks, and in 

 weathering they have initiated valleys 

 and controlled the courses along which 



volcanic injections which have reinforced 

 the weaker slates of which it is mainly 

 composed. The Breidden Hills would long 

 ago ha\"e merged into the Shropshire 

 plain but for the fact that they were once 

 the seat of volcanic activity. It is in 

 the western borders of our islands, and 

 on the edge of the continental shelf, 

 just before it plunges into the abyss of 

 the Atlantic, that earth movements have 

 shown their greatest development. It has 

 always been held that vulcanism and earth 

 folding were in some manner related to 

 each other, hence it is only natural that' 

 in those ])arts where mountain building 

 has been most active we should also find 

 the greatest display of igneous rocks. 

 The form of a mountain must also bear 



