28 GEOLOGICAL HISTORY. [CH. 



In certain more exceptional cases, it is possible to obtain 

 what is technically known as a VERTICAL SECTION ; for example 

 if a deep boring is sunk through, a series of rocks, and the 

 core of the boring examined, we have as it were a sample of 

 the earth's crust which may often teach us valuable lessons 

 which cannot be learnt from maps or horizontal sections. 



It is obvious, that in a given series of beds, which are 

 either horizontal or more or less obliquely inclined, the 

 underlying strata were the first formed, and the upper beds 

 were laid down afterwards. If, however, we trusted solely to 

 the order of superposition in estimating relative age, our con- 

 clusions would sometimes be very far from the truth. 

 Kecent geological investigations have brought to light facts 

 well nigh incredible as to the magnitude and extent of rock- 

 foldings. In regions of great earth-movements, the crust has 

 been broken along certain lines, and great masses of strata 

 have been thrust for miles along the tops of newer rocks. 

 Thus it may be brought about that the natural sequence of a 

 set of beds has been entirely altered, and older rocks have 

 come to overlie sediments of a later geological age. Facts 

 such as these clearly illustrate the difficulties of correct 

 geological interpretation. 



In the horizontal section (Fig. 2), from the summit of Biizi- 

 stock on the left to Saasterg on the right, we have a striking case 

 of intense rock-folding and dislocation ^ Prof Heim^ of Geneva 

 has given numerous illustrations of the almost incredible 

 positions assumed in the Swiss Mountains bj' vast thicknesses 

 of rocks, and in the accompanying section taken from a recent 

 work by Rothpletz we have a compact example of the possi- 

 bilities of earth-movements as an agent of rock-folding. The 

 section illustrates very clearly an exception to the rule that the 

 order of superposition of a set of beds indicates the relative age 

 of the strata. The horizontal line at the base is drawn at 

 a height of 1650 metres above sea-level, and the summit of 

 Biizistock reaches a height of 2340 m. The youngest rocks 

 seen in the diagram are the Eocene beds e, at the base and as 

 small isolated patches on the right-hand end of the section; 

 1 Eothpletz (94). 2 Heim (78). 



