^ INTRODUCTION. 



It is further known, that as there are vast masses of granite and 

 other crystallized rocks, forming the basis on which other rocks are 

 placed, and sometimes piercing into the beds above them in the 

 form of veins; so there are also crystallized rocks, of a different 

 structure, that frequently occur among secondary rocks ; sometimes 

 lying upon or among them in shapeless masses, sometimes intersect- 

 ing them in veins or dykes, and sometimes lying over them, or 

 interstratified with them, in beds of considerable extent and re- 

 gularity. This kind of crystallized rock is known by the names 

 whinstone, basalt, trap, &c. 



In speaking of the relative position of rocks, it is necessary to 

 remark, that those which are the lowest in geological situation, are 

 often, in respect of some portions of them, the most elevated above 

 the horizon. Granite, for instance, which is considered as the low- 

 est rock yet discovered, frequently forms the summits of the highest 

 mountains, the rocks usually super-imposed over it, being arranged 

 in inclined planes on the sides of such mountains. Were all rocks 

 horizontally stratified, the highest in geological position would also 

 be imiformly the highest in horizontal altitude; but it is well known, 

 that the strata lie most frequently in an inclined posture, dipping, as 

 the term is, to one side ; and as the upper beds, instead of being" 

 continued over the lower, are generally broken off, or discontinued, 

 on that side which, owing to the dip, rises highest above the horizon, 

 it is easy to see, that by this means, independent of other breaks and 

 dislocations which frequently occur, the second bed in the series 

 may, if sufficiently prolonged, rise higher than the fia'st, and the third 

 higher than the second ; and the last, or lowest, beds or masses, 

 rising from under the broken edges of the preceding strata, may be 

 protruded to such a height as to overtop them all. Of this we can- 

 not have a better illustration than in the section of the Yorkshire 

 Coast, described in this volume; for here the beds, dipping towai'd 



