154 



GEOLOGY 



that thousands of feet of rock have been gradually 

 removed in the form of detritus. To appreciate 

 this fact some knowledge of structural geology is 

 necessary. In regions which have long been ex- 

 posed to denudation we recognise a very remarkable 

 connection between the configuration of the ground 

 and the nature and mode of arrangement of the 

 rocks. The valleys and low grounds, for example, 

 coincide in a general way with the distribution of 

 the less durable rocks, while escarpments, hills, 

 and ridges mark out the sites of the more resisting 

 rock-masses. Again, in the case of undulating and 

 folded strata, it most frequently happens that anti- 

 clines instead of forming hills give rise to valleys, 

 while synclines correspond as a rule not to valleys 

 but to hills. The reasons are obvious, for relatively 

 hard rocks resist denudation better than softer 

 rocks ; and, while an anticlinal arrangement and 

 the jointing of strata favour the action of the 

 denuding agents, in the case of synclinal strata the 

 rock-structure has just the opposite effect (see 

 LANDSLIPS, MOUNTAINS). Thus the features im- 

 pressed upon the land by denudation depend partly 

 upon the composition and texture of the rocks, and 

 partly upon their structure as rock-masses. In the 

 case of a true mountain-range of recent elevation the 

 larger features of the surface correspond in a general 

 way with the folds of the strata. Thus the mountain- 

 ridges often run in the direction of great anticlinal 

 axes, while the long parallel valleys coincide with 

 synclinal axes (see ALPS). But even in the case 

 of mountains of elevation denudation has often 

 profoundly modified such features. Anticlinal 

 mountains are very unstable ; rock-falls and land- 

 slips from time to time take place ; and the tendency 

 is for all mountains of that character to become 

 effaced. Sooner or later the orographical features 

 change, and are eventually determined by the epi- 

 gene agents, directed and controlled by the com- 

 position and structure of the various rock-masses. 

 Geologists recognise three kinds of mountains : ( 1 ) 

 Mountains of Accumulation, such as volcanoes ; (2) 

 Mountains of Upheaval, such as true mountain- 

 ranges like the Alps ; and ( 3 ) Mountains of Circum- 

 denudation, which owe their origin to the removal 

 of material that formerly surrounded them, such as 

 the mountains of the British Islands. 



A plateau or tableland is simply an elevated 

 plain, and may consist either of approximately 

 horizontal sheets of rock, like the plateau of the 

 Colorado, or of more or less highly folded and even 

 contorted strata, which have been planed down to 

 one general level, like the plateaus of Scandinavia 

 and the Scottish Highlands. Both kinds of table- 

 land are usually traversed by valleys, which have 

 been excavated by running water, and sometimes, 

 as in the case of the Scottish Highlands, they are 

 so highly denuded that their plateau-character 

 becomes obscure. Plateaus owe their elevation 

 to upheaval, those which are built up of horizontal 

 strata being termed plateaus of accumulation, while 

 those which consist of folded and contorted strata 

 are known as plateaus of denudation. Plains are 

 only less elevated plateaus. Some of these, as, for 

 example, the wide alluvial plains and deltas of great 

 rivers, owe their origin to accumulation. Others, 

 again, consist of low-lying land, the level of which 

 has been reduced during a protracted period of 

 denudation. Should such an area eventually be 

 ele vated it would become a plateau of denudation. 



SPECULATIVE GEOLOGY. There are certain great 

 physical problems the data for solving which are 

 more or less incomplete, or in the very nature of 

 things beyond our knowledge. Amongst such is 

 the question of the age of the sun's heat. This, of 

 course, is rather a physical than a geological ques- 

 tion, and yet geology furnishes evidence on the 

 subject which the physicist cannot ignore. Some 



physicists are of opinion that the sun's heat is due 

 to gravitation that, as Sir "NV. Thomson remarks, 

 the sun's matter, before it came together and became 

 hot, may have existed in the condition of two cool 

 solid bodies which collided with the velocity due to 

 their mutual gravitation. If gravitation, therefore, 

 be the only source of the sun's heat, that luminary 

 cannot have been giving out heat at the present 

 rate of radiation for a longer period than 20,000,000 

 years, or, as Professor Tait maintains, 10,000,000 

 years. But no geologist will admit that all the 

 changes that have taken place on the earth's sur- 

 face since the first appearance of life can possibly 

 be included within such narrow limits. Accord- 

 ing to Dr Croll, however, the sun probably origin- 

 ated from the collision of two bodies moving directly 

 towards each other with velocities greater than the 

 velocities due to their mutual gravitation. As the 

 heat generated by the impact of two such bodies 

 would depend upon the velocity possessed by each 

 before collision took place, it is obvious that the 

 energy stored up in our sun may be infinitely 

 greater than that which could have been derived 

 from gravitation alone. So far, therefore, as a 

 possible source of the sun's energy is concerned, 

 Dr Croll is of opinion that life might quite well 

 have begun 100,000,000 years ago. 



Condition of the Earth's Interior. This is another 

 physical problem in the solution of which geology 

 is necessarily interested. Several views have been 

 advanced by physicists, the more generally received 

 opinion being that the earth is a more or less solid 

 globe. Others favour the hypothesis of a thin 

 crust enclosing a liquid or viscous interior ; while 

 yet others think that a liquid substratum separates 

 the crust from a solid nucleus. The appearance of 

 volcanoes and thermal springs shows us that a high 

 temperature exists beneath the crust, and similar 

 evidence of internal heat is furnished by borings 

 and mines. The mean of many observations shows 

 that temperature increases 1 F. for every 54 feet 

 of descent, so that if the temperature at the surface 

 be 50, the boiling-point of water (212) will be 

 reached at the depth of about a mile and a half. 

 It is evident, therefore, that at a comparatively 

 short distance from the surface the heat would be 

 sufficient (at atmospheric pressure) to melt all kinds 

 of mineral matter with which we are acquainted. It 

 is supposed, however, by those who maintain that 

 the earth is solid throughout, that the substance of 

 the earth's interior is kept from liquefying by pres- 

 sure. So far as geological facts go they are opposed 

 to the view of a solid globe or of an enormously 

 thick crust. The folding and contortion of strata 

 seem to imply the presence of an underlying yield- 

 ing mass upon which the solid crust may have a 

 certain freedom to move during the shrinking and 

 contraction that must result from the secular cool- 

 ing of the earth (see EARTH, MOUNTAINS). 



The origin of volcanic action has also been a 

 much-canvassed question, and is variously ex- 

 plained according as the hypothesis of a solid or of 

 a viscous interior is held to be the more probable 

 (see VOLCANOES). Closely connected with such 

 problems is that of the origin of oceanic basins and 

 continental areas. Of late years the belief has 

 gained ground that these dominant features of 

 the earth's surface are of primeval antiquity 

 that in their origin they antedate the oldest of 

 the sedimentary formations. It is a remarkable 

 fact that hitherto, amongst the various formations 

 that enter into the composition of the land of the 

 globe, no trace of any abysmal accumulations has 

 been met with. On the contrary, the aqueous 

 rocks appear to have been deposited as a rule in 

 relatively shallow seas. Many oscillations of level 

 have taken place at successive periods within each 

 continental area, by which the extent and outline 



