GEOLOGY 



THE ARRANGEMENT OF STRATI- 

 FIED ROCKS, FOLDS, AND FAULTS. 

 When beds are laid down in a 

 regular succession, each sheet hori- 

 zontally upon that below it, the 

 series is said to be conformable. 

 But if one series of beds has been 

 tilted and new beds are deposited 

 across its worn edges, the two 

 series are unconformable. Uncon- 

 formities are important because 

 they indicate long intervals of time 

 during which no beds were deposi- 

 ted ; and there are some complete 

 gaps in geological history when a 

 universal unconformity marks a 

 time of world-wide disturbance of 

 the crust. The recognition of un- 

 conformities is also important in 

 applied geology, for if their exist- 

 ence be overlooked serious errors 

 may be made in mining or in 

 searching for minerals or water. 



The stratified rocks of the earth's 

 crust were laid down in layers 

 which were originally horizontal ; 

 the bedding may have been regular 

 where the material was deposited 

 or rearranged by strong currents. 

 The beds, however, are generally 

 tilted, and their slope is known as 

 the dip. Rocks are also disturbed 

 by folding. The rocks may be bent 

 by upfolds in arches, which are 

 known as anticlines ; the beds may 

 sag in downfolds into troughs or 

 synclines. Such folds are often due 

 to lateral pressure, as when a table- 

 cloth is wrinkled into folds by 

 being pushed across a table. When 

 the lateral movement is consider- 

 able, folds may be so crowded to- 

 gether that the two sides of each 

 fold may be parallel as in a closed 

 concertina. Such compressed folds 

 are known as isoclines. If the 

 plane of the isocline, instead of 

 being vertical leans over to one 

 side, the beds on the under side are 

 turned upside down, and the suc- 

 cession is said to be reversed. An 

 upfold around a point forms a 

 dome, and a downfold around a 

 point forms a basin. 



Faults and Sunklands 



The disturbances of the crust 

 often produce fractures on one 

 side of which the beds may be dis- 

 placed upward or downward. Such 

 movements are known as faults. 

 They break the continuity of beds, 

 so that a sheet of stone may end 

 abruptly at a fault, beyond which 

 it may lie above or below its 

 original level. In mining it is there- 

 fore important to recognize faults, 

 and to determine on which side 

 the beds have been pushed up- 

 ward (the upthrow side), and on 

 which they have sunk (the down- 

 throw side). Faults may occur 

 singly or in series ; several faults 

 with the throw all in the same 

 direction are known as step-faults. 



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A pair of parallel faults, between 

 which the beds have moved down- 

 ward, forms a trough fault. A 

 pair of parallel faults between 

 which the rocks have been left up- 

 raised form a ridge fault, and the 

 block of country between them is 

 a horst. A valley formed by the 

 sinking of a strip of country be- 

 tween two parallel faults is a rift 

 valley, while a large tract of 

 country which has sunk within a 

 ring of faults is a sunkland. The 

 continents may be regarded as 

 vast horsts which have been left 

 upstanding owing to the subsi- 

 dence of the ocean basins by com- 

 bined faulting and folding ; the 

 oceans cover the largest of the 

 sunklands, whose floors have 

 foundered beneath the sea. 

 Submerged Continents 



According to a once popular 

 theory, the ocean basin and conti- 

 nental elevations have been in the 

 main permanent throughout geo- 

 logical times. The balance of 

 evidence, however, is against this 

 view. Some portions of the conti- 

 nents have, it is true, remained 

 above sea level throughout geo- 

 logical time, and it is correspond- 

 ingly probable that parts of the 

 ocean basins may have been per- 

 manently below sea level. Never- 

 theless, there has been great inter- 

 change of ocean and continent. 

 There is evidence, for example, of 

 a great continent, known as Gon- 

 wanaland, which once extended 

 from Australia westward across 

 the Indian Ocean, included most 

 of India and Africa, continued 

 across the Atlantic, and comprised 

 the eastern highlands of South 

 America. 



EARTH MOVEMENTS AND MOUN- 

 TAINS. The movements in the 

 crust which cause faults some- 

 times tend to pull the rocks asun- 

 der as if the crust were shrinking, 

 while at others the rocks are 

 pressed together. Faults of the 

 former class are nearly world-wide 

 in distribution, whereas those due 

 to compression on a great scale 

 at any one time in the earth's 

 history have been restricted to 

 particular belts along which the 

 rocks have been crumpled into 

 mountain chains. 



Some of the continental high- 

 lands may be regarded as vast 

 horsts which have been left up- 

 standing while the surrounding 

 countries have been lowered by 

 folds and faults. Such horsts form 

 the peninsula of India, the plateau 

 of Western Australia, the highlands 

 of Brazil and tropical Africa. In 

 contrast to these broad highlands 

 are the long and comparatively 

 narrow chains of fold-mountains, 

 which are due to the folding of 



GEOLOGY 



strips of the earth's crust along 

 lines of special compression. Such 

 fold-mountain systems are repre- 

 sented by the Andes in S. America, 

 the Alps, and the Himalayas. 



These fold-mountain chains have 

 been produced at successive periods 

 of active disturbance of the earth's 

 crust. Three periods of the forma- 

 tion of fold-mountains have been 

 of special importance : that which 

 has left the greatest mark on the 

 existing topography of the earth 

 produced in Europe, in compara- 

 tively late geological times (oligo- 

 cene and miocene), the Alpine 

 system which includes the Alps, 

 Pyrenees, Apennines, and the main 

 chain across the Balkan Peninsula. 



As to the ultimate cause of 

 such earth-movements there is no 

 full agreement. The most obvious 

 cause is the adaptation of the 

 crust to the shrinkage of the in- 

 terior. The fold-mountain chains 

 are analogous to the wrinkles 

 formed on the skin of an apple by 

 the shrivelling of the pulp ; and 

 with additions to explain the re- 

 striction of the folding to special 

 belts this explanation is probably 

 the most satisfactory. The graat 

 subsidences between the folded 

 belts are probably due to the 

 shrinkage of the interior, leaving 

 areas unsupported. 



Movements of the Earth's Crust 



The upward and downward 

 movements of the crust have de- 

 termined the main configuration of 

 the earth, but many secondary 

 geographic features are also due 

 to the heaving of the earth's sur- 

 face. The crust is in a state of con- 

 tinual tremor and movement ; 

 large parts of the earth's surface 

 are in such delicate equilibrium 

 that the weight of a fresh layer of 

 sediment, or of extra water at high 

 tide, or even a heavy storm of rain, 

 may press down the loaded area. 

 Similarly the removal of a layer of 

 material from a land may not per- 

 manently lower the surface, as the 

 land may rise owing to the lighten- 

 ing of its load. This ready yielding 

 by the surface to slight variations 

 in weight may appear inconsistent 

 with the existence of mountain 

 chains and the ocean basins. It 

 might be thought that mountains, 

 by pressing down their founda- 

 tions, would sink to the average 

 level, while the ocean floors would 

 rise to it. The existence of moun- 

 tains is explained by the weight of 

 their raised masses being compen- 

 sated by a deficiency of material 

 in their foundations. This princi- 

 ple is known as isostacy. 



According to it all blocks of the 

 crust which are equal in area, and 

 extend down to a surface about 

 70 m. below sea level, are equal in 



