8 Journal of Travel a7id Natural History 



occurrence in nature than this. What in a geological sense is an 

 upward fold or arch, instead of rising as such above the surface, and 

 forming a hill or ridge, is very often a valley; while, on the other 

 hand, a downward fold or basin of the rocks, which it might have 

 been conjectured would have produced a valley, rises even into a 

 bulky mountain. The examples where a hill has its form determined 

 by a dome-shaped fold of the strata, or where a valley has been 

 formed by a trough-shaped depression of the strata, are of much less 

 common occurrence, and when they do occur, there is usually some 

 other circumstance which has come in to aid the guiding influence 

 of geological structure, such, for instance, as the greater hardness 

 of the rocks that form the dome, or the greater softness of those 

 that have been bent into the trough. And thus the seeming para- 

 dox remains true, that what should be, by geological structure, 

 mountains, have been turned into valleys, while what ought to have 

 been valleys, rise up into mountains. The attempt, therefore, to ex- 

 plain the present outlines of the ground by reference mainly to 

 former crumplings of the rocky crust of the earth must be aban- 

 doned. 



Nor can we more successfully appeal to the many masses of 

 igneous rock which traverse the stratified formations, as causes of 

 our existing hills. These rocks have, indeed, been forced up- 

 wards from a heated region within the earth's crust. But it can 

 be shewn that many of them never reached the surface at all at 

 the time of their ejection, that those which did flow out as melted 

 rock at the surface were subsequently deeply buried under later 

 formations, and that the actual appearance of igneous rocks above 

 ground is due, as will be pointed out a little further on, to the 

 removal of these later formations, and the consequent exposure of 

 the trappean masses that lay beneath. Take, as illustrations, the hills 

 which rise from the north side of the Firth of Tay and stretch 

 north-eastwards through the chain of the Sidlaws ; and also the hills 

 on the opposite side of the Firth, which range through Fife and 

 the line of the Ochils to Stirling. These heights consist mainly 

 of different beds of trap-rock — old lavas which were poured over 

 the surface from a succession of volcanic vents. They are inter- 

 stratified with beds of sandstone and other sedimentary deposits 

 by which they are shewn to have been erupted at intervals, and 

 more or less horizontally during the time of the lower old red 

 sandstone. From the subjoined section it will be seen that their 

 former level position has been thrown into an inclined one. The 



