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THE POPULAR SCIENCE MONTHLY. 



terrancan forces, by a very simple considera- 

 tion. It is certain that during, the enormous 

 periods of time of which the records have 

 been discovered by the geologist, there have 

 always been continents and oceans upon the 

 earth's surface, just as at present, and it is 

 almost equally certain that the proportions of 

 the earth's surface occupied by land and water 

 respectively have not varied very widely 

 from those which now prevail. But, at the 

 same time, it is an equally well-established 

 fact that the denuding forces ever at work 

 upon the earth's surface would have been com- 

 petent to the removal of existing continents 

 many times over, in the vast periods covered 

 by geological records. Hence we are driven 

 to conclude that the subterranean movements 

 have in past times entirely compensated for 

 the waste produced by the denuding forces 

 ever at work upon our globe. But this is not 

 all. The subterranean forces not only pro- 

 duce upheaval ; in a great many cases the 

 evidences of subsidence are as clear and con- 

 clusive as are those of upheaval in others. 

 Hence we are driven to conclude that the 

 forces producing upheaval of portions of the 

 earth's crust are sufficient, not only to balance 

 those producing subsidence, but also to com- 

 pensate for the destructive action of denuding 

 agents upon the land-masses of the globe. 



It is only by a careful and attentive study 

 and calculation of the effects produced by the 

 denuding agents at work all around us, aided 

 by an examination of the enormous thick- 

 nesses of strata formed by the action of such 

 causes during past geological times, that we 

 are able to form any idea of the reality and 

 vastness of the agents of change which are 

 ever operating to modify the earth's external 

 features. When we have clearly realized the 

 grand effects produced on the surface of the 

 globe by these external forces, through the 

 action of its investing atmosphere and circu- 

 lating waters, then, and only then, shall we 

 be in a position to estimate the far greater 

 effects resulting from the internal forces, of 

 which the most striking, but not the most 

 important, results are seen in the production 

 of volcanic eruptions and earthquake-shocks. 



Another series of facts which serves to con- 

 vince the geologist of the reality and potency 

 of the forces ever at work within the earth's 

 crust, and the way in which these have op- 

 erated during past geological periods, is found 

 in the disturbed condition of many of the 

 stratified rock-masses <>i'\\ hich it is composed. 

 Such stratified rock-masses, it is clear, must 

 have been originally deposited in a position 

 of approximate horizontality ; but they arc 

 n >w often found in inclined and even vertical 



positions ; they are seen to be bent, crum- 

 pled, puckered, and folded in the most 

 remarkable manner, and have not unfre- 

 quently been broken across by dislocations 

 "faults" which have sometimes dis- 

 placed masses, originally in contact, to the 

 extent of thousands of feet. The slate-rocks 

 of the globe, moreover, bear witness to the 

 fact that strata have been subjected to the 

 action of lateral compression of enormous 

 violence and vast duration ; while in the 

 metamorphic rocks we see the effects of still 

 more extreme mechanical strains, which have 

 been in part transformed into chemical ac- 

 tion. No one who has not studied the crushed, 

 crumpled, fractured, and altered condition 

 of many of the sedimentary rocks of the 

 globe, can form the faintest idea of the enor- 

 mous effects of the internal forces which have 

 been in operation within the earth's crust 

 during earlier geological periods. And it is 

 only by such studies as these that we at last 

 learn to regard the earthquake and volcanic 

 phenomena of our globe, not as the grandest 

 and most important effects of these forces, 

 but as their secondary and accidental accom- 

 paniments. 



Professor Judd here passes to a very 

 lucid presentation of the later views 

 of geologists in regard to the mode of 

 origin and development of mountains. 

 This is an important part of his discus- 

 sion, hut we have no room for it here. 

 He concludes : 



From what has been said, it will be seen 

 that mountain-chains may be regarded as 

 cicatrized wounds in the earth's solid crust. 

 A line of weakness first betrays itself at a 

 certain part of the earth's surface by fissures, 

 from which volcanic outbursts take place; 

 and thus the position of the future mountain- 

 chain is determined. Next, subsidence during 

 many millions of years permits of the accu- 

 mulation of the raw materials out of which 

 the mountain-range is to be formed ; subse- 

 quent earth-movements cause these raw ma- 

 terials to be elaborated into the hardest and 

 most crystalline rock-masses, and place them 

 in elevated and favorable positions; and 

 lastly, denudation sculptures from these 

 hardened rock-masses all the varied mountain 

 forms. Thus the work of mountain-making 

 is not, as was formerly supposed by geolo- 

 gists, the result of a simple upheaving force, 

 but is the outcome of a long and complicated 

 series of operations. 



The careful study of other mountain- 

 chains, especially those of the American Con- 



