R. M. Deeley — Mountain Building. Ill 



althougli judging from the fossils obtained, no higher sub-zones 

 were present than usually occur in Lincolnshire. The change in 

 thickness of the Upper Lias from Lincoln southwards, moreover, is 

 not due to an increase in the thickness of the component zones, which 

 vary in thickness as pointed out above, but is due to the greater 

 extent of the non-sequence at the top of the Lias when traced 

 northwards across Lincolnshire; thus, while the Jihilatum, hraunianuniy 

 and lilli sub-zones are present in Northamptonshire, of these only 

 the fihdatum sub-zone is definitely represented at tirantham, and 

 none of them are found at Lincoln. It appears, therefore, that 

 uplift in South Yorkshire occurred at intervals, namely, during the 

 deposition of Lower and Middle Lias, and towards the close of 

 deposition of Upper Lias. 



III. — Mountain Building. 

 By K. M. Deeley, M.Inst.C.E., V.P.G.S. 

 rpHE structure of mountain ranges has always been difficult to 

 J_ understand. They often show that peculiarly complicated 

 disturbances of strata have occurred in the process of their formation. 

 Mountain ranges in many stages of dissection are to be seen in 

 various parts of the world; but the better knowledge which their 

 study has furnished us with has not, at the moment, always assisted 

 us in the better understanding of the problem of mountain building. 



At the present time the compression theory may be said to be the 

 one most generally accepted. It is thus described by James Geikie : ' 

 " Little progress could be made towards a satisfactory theory until 

 the geological structure or architecture of individual mountain chains 

 had been studied with precision. Many observations and descriptions 

 of the folded rociis of the Alps and other regions had been recorded 

 . . . but . . . geology could still present no clear conception of 

 a mountain range as an organic unity ... it was not until the 

 appearance in 1843 of the well known essay by Professors W. B. 

 and H. D. Rogers on the physical structure of the Appalacians, that 

 geologists generally began to realize what is meant by the architecture 

 of mountains of elevation. Thanks to the labours of these brilliant 

 observers and their many successors, we are no longer in doubt as to 

 the part played by compression in the formation of mountain ranges." 

 That compression is the cause of the upheaval of mountain ranges, 

 and the folded structure they present, James Geikie had no doubt, 

 and he enforces his argument by pointing to the phenomena of 

 cleavage, schistosity, etc., as the result of the same action. 



To some, however, the amount of compression required to form 

 a mountain range, not to mention the sharply marked anticlines and 

 synclines of less elevated regions, seems greater than can be allowed. 

 To again quote James Geikie,' "While overfolding and wholesale 

 horizontal displacements are the most characteristic features of 

 Alpine architecture, it must not be forgotten that compression 



' Mountains, their Origin, Growth, and Decay, 1913, p. 66. 

 2 Ibid., p. 130. 



