GROWTH OF GEOLOGY. 259 



cations, since it induces radial and tangential 

 strains. The theory that volcanic eruptions count 

 for much in mountain-making is a superficial ex- 

 aggeration. The architecture (Tektonik) of moun- 

 tains must be studied in 'detail. They have a one- 

 sided structure — in the Alps, the Balkan, the Cau- 

 casus, and Ararat — all expressions of a tangential 

 force from south to north in Europe, and towards 

 the south in Asia. But besides the dislocations of 

 the lithosphere there have been great transgressions 

 and regi-essions of the hydrosphere, not less momen- 

 tous than the rise of mountain chains. The conti- 

 nents, as Shaler said, are due to contractions of the 

 whole crust, while mountains are due to foldings of 

 the outer layers in consequence of contractions in 

 the deeper. But, just as in pack ice, there may be 

 unyielding masses, which have to be piled one upon 

 the other, or may be simply undisturbed and over- 

 lapped. 



RECOGNITION OF ICE AGES. 



Evidences of Glaciation. — In a suitable area, such 

 as Scotland, every beginner in geological study is 

 familiar with the smoothed contours of rocks, the 

 striated surfaces, the " crags and tails," the boulder- 

 clay and so on, which prove the former presence of 

 enormous glaciers, and that at no very distant date. 

 Many of the phenomena are obvious and they were 

 of course familiar to Hutton and his friends. But 

 they received other interpretations than that which 

 seems to us almost self-evident — now that the riddle 

 has been read. They were explained as due to 

 floods of water and strong tides, and these were 

 again explained by supposing elevations or depres- 

 sions wherever they were required. 



