iv 



INTRODUCTION. 



r I "'HE existing geographical and physical contours of 

 J_ the British Islands are the outcome of the long and 

 varied geological history which this particular part of the 

 world possesses. There are few other regions of the earth's 

 surface which in so small an area exhibit so many dif- 

 ferent systems of rocks as are to be found in the British 

 Islands. England alone includes portions of nearly all 

 the rock-systems which are found on the continent of 

 Europe. 



Each of these rock-groups was formed during the main- 

 tenance of certain physical and geographical conditions, and 

 the restoration of the particular conditions which prevailed 

 during any one period will reveal one phase in the geo- 

 graphical evolution of Britain. Further, in the multi- 

 plicity of rock-groups we have proof that changes in the 

 physical conditions of the area have been frequent, and it 

 becomes evident that the history of this evolution is a 

 long one ; it is, in fact, a history of alternate upheaval 

 and depression, of the repeated formation of islands and 

 continents, and of their subsequent detrition and submer- 

 gence. In this long succession of changes there have been 

 many different arrangements of land and sea over the 

 area where the British Isles now stand ; every part of our 

 country has been repeatedly depressed beneath the sea, 

 though some parts have been submerged much more fre- 



