574 MULL. GEOLOGY. 



of one horizontal bed over another ; and where the decli- 

 vity is the result of vertical fractures, by descending steps, 

 either of each successive stratum, or of a more extensive 

 series. In this latter case the boundary of a mountain 

 is either a single vertical cliff or a succession of two or 

 more such; some gentle descents being almost always 

 found among these precipices, from the slopes generated 

 by the fallen fragments. The subjoined diagram* will 

 give a better notion than words of the forms in question: 

 its outline is in fact almost a representation of the moun- 

 tain which constitutes the promontory of Gribon. In 

 examining the positions of the strata which form the 

 primary, and even many of the secondary mountains, it 

 will be found that the beds which enter into their com- 

 position are rarely if ever continuously horizontal. They 

 are sometimes bent, at others partially elevated, or now 

 and then even vertical, as represented in the other 

 diagrams ;f the existence, as well as the declivities of the 

 hills appearing to have been the result, either of an elevating 

 force, or of a subsidence, by either of which the appear- 

 ances might have been produced. But the absolute 

 horizontality of the trap beds shows that these remain 

 in the position in which they were first formed. The 

 period of this formation is obviously posterior to that 

 of the latest stratified substances with which we are 

 acquainted, since this class of mountains is found cover- 

 ing those substances even to a considerable depth. That 

 no general disturbance and fracture of the earth's surface 

 has taken place since this period, is proved by their free- 

 dom from dislocation and change of position. It is diffi- 

 cult to comprehend by what means horizontal deposits 

 of so partial a nature are found occupying positions so 

 elevated. But to whatever causes their formation may 

 be assigned, it is at least evident that their origin, recent 



* Plate XX. fig. 1. f Plate XX. fig. 2, 3, 4, 5. 



I I must here perhaps except some of the most recent partial forma- 

 tions. 



