ME CADELL ON EXEEEIMENTAL EESEAECHES IN MOUNTAIN BUILDING. 347 



the whole mass to rise and glide forward over the lower and less disturbed beds along 

 a major thrust-plane, inclined at a very slight angle to the horizon. During the forward 



Fig. 13. 



movement the friction tended to retard the front more than the back of the advancing 

 mass, and make the inclination of the thrust parts of the section still greater. Had the 

 experiment been continued, the originally horizontal parts of the thrust mass would no 

 doubt have reached verticality, and then begun to bend in towards the major thrust- 

 plane below, just as the fingers of an outstretched hand, when pushed along a table, tend 

 to turn in and fold back on the palm.* Figs. 12 and 13 are taken from photographs. 



Fig. 14. 



Fig. 14 is a sketch of a very characteristic section which was not photographed. 

 After continued piling up, such as is shown in fig. 3, the heap of strata rose and slid 

 forward over a major thrust-plane, against which the smaller thrusts are all truncated. 

 It is obvious that beds repeated in this way in nature, without inversion or folding, might 

 come to have an appearance of enormous thickness, and thus greatly mislead the field 

 geologist. It might indeed be almost impossible to arrive at a correct conclusion as to the 

 thickness of such a formation, were the underlying major thrust concealed, and nothing 

 but the edges of the upper beds exposed to view. This structure is now known to be 

 of common occurrence in the North- West Highlands, and these experiments show clearly 

 on what mechanical principles many of the extraordinary aDd remarkably deceptive 

 relationships of the rocks of that region may be explained. This experiment shows that 

 underneath a series of beds, repeated and heaped together by small thrusts, inclined 

 perhaps at considerable angles, there runs in the majority of cases a major thrust or 



* This has often taken place in nature. See sections through Ben More, Assynt, &c, figs. 15, 16, 17, Quart. Jour. 

 Geol. Soc, 1888, pp. 421, 423. 



