206 



K NOWLEDGE 



[NOVKMBER 1, 1898. 



E, accumulating in the sea, B, are derived. For manifest 

 reasons, these will be thickest and coarsest near land, 

 where the carrying power of water is at its greatest. Now, 

 OR the reasoning already given, this accumulation will 

 finally lead to upheaval, the uprise of strata being; greatest 

 in the region where the " blanket " is thickest. That is to 

 say, the base is tilted. The strata consequently glide sea- 

 ward and pucker up upon the tilted base (Fig. 7). ]\Iean- 

 while the continental mass (A) is continually undergoing 

 denudation, and the rocks immediately beneath, therefore, 

 are cooling. We may say that the young land to the righo 

 is pulling the blanket off its older neighbour, the area A. 

 The cooling of A causes a subsidence and faulting, and the 

 faulting, weakened, and sinking crust is there least able to 

 resist eruptive material, so that at last (Fig. H) a volcauic 

 chain, F F F, may grow up behind the fold chain. 



This, briefly, is the story suggested by Prof. Eeyer, 

 a story also fairly consistent with existing mountain struc- 

 tures. But it need not be regarded as a theory absolutely 

 opposed to that so clearly propounded to the English 

 student by Prof. Lapworth. The heating effect of deposi- 

 tion suggested by Mr. Mellard Eeade, the crust contraction 

 to which Prof. Lapworth gives prominence, the " gliding '" 

 of Prof. Reyer, are all causes that must operate. Prof. 

 Pieyer's theory may explain many cases of folding, Mr. 

 Mellard Reade's many cases of upheaval, and yet the 

 great wrinkles on the face of Mother Earth may be due to 

 her withering as the warmth of her youth departs from 

 her. 



Clearly, from what has been said, volcanic phenomena 

 are a mere incident in the growth of a mountain chain. 

 They do not, for instance, appear to have played a leading 

 part in Alpine history, and the Rocky Mountains were 

 already elevated before the great trachytic and basaltic 

 outflows of that region occurred. Volcanic forces cannot, 

 therefore, for one moment be regarded as standing in a 

 causative relation to mountain buildmg. Nevertheless, in 

 the Andes and the Himalayas the abundant presence of 

 volcanoes is food for thought. However, the question of 

 the causes of volcanic action scarcely belongs to this 

 paper. 



Here we may allude to a third feature of mountain 

 structure. It emphasizes the enormous pressures to which 

 the folded rocks were subjected. It is the alteration of the 

 microscopic structure of these rocks. 



We find, for instance, clays, with all their once higgledy- 

 piggledy particles, twisted round into a direction at right 

 angles to the force of compression, so that they can be 

 split up into lamiuie, and are no longer clays but slates. 

 Limestones lose the traces of their organic relics, and 

 become recrystallized as marble. Some rocks are seen with 

 their constituent minerals literally crushed and rolled over 

 and into one another, as though they had been through a 

 colossal crushing mill (Mylonitic structure). The quartz of 

 granite, for iiihtance, is powdered, the felspar cracked and 

 reduced to cloudy particles, the mica twisted and shredded. 

 The rock has also been, as it were, mustk-uted m the 

 presence of in-soaking water. Old minerals have been 

 dissolved out, fresh ones formed. 



In some cases a parallel order of the minerals has been 

 induced. It is as if the rock had become plastic under 

 these stupendous stresses, and that we had here its lines 

 of flow. Nothing could be more eloquent of the irresistible 

 nature of the mountain-making forces. It is interesting, 

 too, to notice how we have thus repeated, in a thin flake of 

 rock that would scarcely weigh a grain, the same story of 

 enormous lateral pressure that we find in considering the 

 stratigraphical structure of an Alpine massif. 



To summarize our deductions, we have in the history of 



every great mountain chain the following phases. We 

 can do without any appeal to " old Seismos " now to 

 account for the elevation. A long period of quiet subsidence 

 and deposition of sediment is followed by upheaval. There 

 is a process of lateral compression relieved by a bulging, 

 the formation of a ridge or ridges, with troughs on either 

 side. Probably there are no great paroxysms ; the steady 

 squeezing and upward creep goes on day by day, year by 

 year, age by age. Strata are imperceptibly thrown into 

 bends, into loops, the foldings are heaped up one above 

 the other, overfolds are formed. The rising mass slowly 

 becomes a prominent terrestrial feature. Stresses, cul- 

 minating day by day, are at last relieved by the formation 

 of faults and thrust planes, and as the ruptured strata 

 slip there are earthquakes. Rocks are crushed and 

 metamorphosed, softened, moulded, possibly even liquefied. 

 There may be volcanic outbursts along the axis or upon 

 the margins of the rising area. 



The emerging mass becomes subjected to denudation. 

 In the main troughs which will be sinking beside the rising 

 ridge, forming seas or lakes, sediments will accumulate. 

 Presently these areas cease to subside and become involved 

 in a greater movement of elevation, as is shown by the 

 Swiss molusse and the Siwalik rocks on the Himalaya 

 flanks. So the vast growth continues. Strata tilted on 

 its rising shoulders slide and are crumpled. Above, the 

 snow and glacier are soon at work — unequal heating by 

 day and frost by night, rain and wind, splinter the 

 metamorphosed upturned rock masses into peak and 

 pinnacle, cirque and precipice. Thus in the course of ages 

 the mountain chain attains its prime, and a brief equilibrium 

 follows. 



But the forces of lateral pressure and upheaval are dying 

 away or they have found a weaker area elsewhere. The 

 volcanoes become extinct, the earthquakes less violent 

 and less frequent. Every moment a hundred streams 

 carry away their quota of material stispendtd or dissolved. 

 So the period of decay sets in. From the still eruptive 

 Himalayas we may turn to the more quiescent Alps, from 

 there again to the worn-down masses of Scandinavia and 

 Scotland, from there to the still more ancient mountain 

 range half buried beneath the strata of Wales and Central 

 England ; and so the story ends at last as it began, in 

 sedimentation in the sea. 



In conclusion, attention may be called to the rather 

 remarkable fact that in the moon no great meridional 

 mountain ridges, such as one might expect from the analogy 

 of the earth, are to be traced. Neither have the mountains 

 of the moon so distinctly the linear ridge-like arrangement 

 characteristic of terrestrial mountain axes. One might 

 have anticipated, on the contrary, in the absence of an 

 atmosphere and atmospheric denudation and with feebler 

 gravitational attraction, that broad regions of crust folding 

 would have been more conspicuous than on the earth. It 

 may be that these features have been mask(>d by the 

 subsequent precipitation of the lunar atmosphere ; but the 

 volcanic character of lunar scenery is hardly consistent 

 with this hypothesis. This, however, is a question for the 

 astronomer to consider. 



Utttcrs. 



[Thn Editor does not hnld himself responsible for the opinions or 

 statements of correspondents.] 



♦- 



To the Editor of Knowledge. 



Dkak Sir, — The arguments which you have brought to 

 bear upon the significant question, " What is the Sun'g 



