834 EEPOBT— 1893. 



FRIDAY, SEPTEMBER 16. 



The following Papers and Reports were read :— 



1. On the Limits letween Physical Geography and Geology} 

 By Clements R. Markham, C.B., F.B.S. 



2. On the Helations of Geology to Physical Geography. 

 By W. ToPLET, F.B.8., Geological Survey of England. 



Professor Lapworth in tis address to the Geological Section last year at 

 Edinburgh showed clearly the close relationship of geology to physical geography 

 as regards the larger features of the earth's surface — the structure and distributioa 

 of mountain chaius, the great folds which traverse the earth's crust, and the wider 

 areas of uplift and depression. 



I propose to refer mainly to the minor features of the surface, and to show that 

 the nature and 'lie 'of the rocks determine these surface features. From this 

 point of view geology forms the basis of physical geography, and a geographer 

 must necessarily be to some extent a geologist. 



There is no need to point out that the converse of this is equally true. A 

 geologist who would understand in ■^hat way existing geological conditions have 

 been brought about — how strata have been deposited, volcanic rocks erupted, and 

 how denudation has carved the surface into its present shape — must study similar 

 phenomena now in progress. 



A comparison of geological and physical maps of any area at once shows that 

 different rock-groups and formations present different types of land, the hills and 

 mountain chains coinciding with the outcrop of certain rocks or strata, whilst other 

 formations are characterised by plains or by lowlands. It is thus clear that the 

 geological structure of any district determines its physical geography. This relation 

 is equally apparent when we compare the structure and surface features in detail. 

 The older masses of rock frequently form mountainous land ; the newer Palaeozoic 

 and the Secondary rocks occur mainly as a succession of plains and escarpments, 

 each determined by the outcrops of certain beds. 



The history of valleys, plains, and gorges can only be understood by reference 

 to their geology. We then see why rivers, after for a while running over wide open 

 plains, suddenly break through hill ranges, cutting the escarpments at right angles, 

 the explanation being that the streams began their work when the whole formed 

 a comparatively even surface, the existing features being due to long-continued 

 erosion. 



The escarpments, plains, and transverse river valleys of central and southern 

 England and of eastern France form excellent examples of this structure. Here 

 the geology is simple, a fairly continuous dip in one direction and different beds 

 cropping out beneath each other in definite order. 



On the flanks of mountain chains this simple structure does not hold good ; 

 there the beds are frequently contorted, inverted, and thrust over each other, so 

 that a superficial reading of the geology would give erroneous results as to the 

 order of succession. But even here the present ' lie ' of the beds has determined 

 the physical geography, the disturbances of the rocks having been produced when 

 they were deeply covered by other strata now removed by denudation. 



Some igneous rocks weather into conical hills resembling volcanic cones. The 

 same thing often occurs in the weathering of loose sands, whilst sand is frequently 

 blown into conical and crater-shaped hills. A hasty glance at the outward shape 

 of such hills might mislead a traveller. Many conical hills in the south of Scotland 

 are old volcanic vents, up which molten lava once came ; but the volcanic cones 

 have been long since removed by denudation, and what we now see are only the 



' Published in the Scot. Geog. Mag., is. ri893), pp. 63.3-638. 



