8 Eev. 0. Fhher — Convection Currents in EartWa Interior. 



" rost-Cambriau Igneous Rocks of older date than the Great Thrust- 

 Movements of the Region : their Petiography," pp. 440-r)2, 

 "Contact Metamorphismof the Camhrian Dolomites and Limestones," 



pp. 453-62. 

 "The Eastern Schists." General Description (witli J. Ilorne), 

 pp. 595-601. 



2. District Memoirs. 



1897. " Geology of Cowal." General Bemarks on the Petrography of the Cowal 



District, pp. 295-9. Description of an Albite Schist, pp. 299-300. 



3. iiltcet Memoirs. 



Ireland. 



1891. Sheets 31 (part) and 32 (parts of Donegal, etc.). 



Scotland. 

 1896. Sheet 5 (part of Kirkcudbrightsliire). 



Sheet 75 (parts of Aberdeen, Banff, etc.). 

 1905. Sheet 37 (Mid- Argyll). 

 1907. Sheet 20 (Islay, etc.). 



England and "Wales. 

 1902. Sheet 325 (Exeter). 



1904. Sheets 355 and 356 (Kingsbridge and Salcombe). 

 1907. Sheet 353 (Mevagissey). 



II. — On Convection Currents in the Earth's Interior. 

 By Rev. 0. Fisher, M.A., F.G.S. j, 



I'E this Magazine for jS'ovember Mr. Mellard Reacle referred to the 

 difficulty of imagining the adequacy of any known force to 

 accomplish such tremendous effects as to produce overthrusts like 

 those which have been lately described as occurring in the IS'orth- 

 "West Highlands,^ and he consequently appears to be a little doubtful 

 of the reality of the phenomenon. I think, however, that the con- 

 clusion of so many competent field geologists must be accepted, and 

 that our theories of mountain-building must be made to fit the facts 

 rather than the facts the theories. 



An overthrust is only one of the ways in which a region of the 

 earth's crust may be compressed into a smaller area by horizontal 

 pressures acting in opposite directions, and enormous as the stress 

 must be, it does not seem that it need be greater to cause an 

 overthrust than what would be required to produce the folding 

 and consequent elevation, which is the more usual manner in which 

 the compressing force has been satisfied ; for if we suppose the mean 

 elevation of a tract produced to be the same in the two cases, the 

 work against gravity would be the same, and what we have to 

 compare is, in the one case the friction along the thrust-plane, and 

 in the other the bending and fracturing of the rigid rocks. It is not 

 obvious that the former need require more force than the latter. 



1 have elsewhere endeavoured to explain the mechanism of an 

 overthrust by the analogy of what may be observed in the behaviour 

 of ice on a frozen river." If the edge of a piece of floating ice gets 



^ Survey Memoirs, TJie Geological Striictmr of the N. W. Eifjhlands of Scotland, 

 1907. 



2 Physics of the Earth's Crust, 2nd ed., p. 321. 



