the great beds of clays, marls, and limestones afford evidence of these destructive processes continued into Pliocene times.  We have already referred to Schmidt’s estimate that the sedimentary covering must have in places amounted to from 15,000 to 20,000 metres.  The evidence for this is mainly tectonic or structural; but is partly forthcoming in the changes which the materials now open to our inspection plainly reveal.  Thus it is impos-

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sible to suppose that gneissic rocks can become so far plastic as to flow in and around the calcareous sediments, or be penetrated by the latter—­as we see in the Jungfrau and elsewhere—­unless great pressures and high temperatures prevailed.  And, according to some writers, the temperatures revealed by the intimate structural changes of rock-forming minerals must have amounted to those of fusion.  The existence of such conditions is supported by the observation that where the.crystallisation is now the most perfect, the phenomena of folding and injection are best developed.[1] These high temperatures would appear to be unaccountable without the intervention of radiothermal effects; and, indeed, have been regarded as enigmatic by observers of the phenomena in question.  A covering of 20,000 metres in thickness would not occasion an earth-temperature exceeding 500 deg.  C. if the gradients were such as obtain in mountain regions generally; and 600 deg. is about the limit we could ascribe to the purely passive effects of such a layer in elevating the geotherms.

Those who are still unacquainted with the recently published observations on the structure of the Alps may find it difficult to enter into what has now to be stated; for the facts are, indeed, very different from the generally preconceived ideas of mountain formation.  Nor can we wonder that many geologists for long held

[1] Weinschenk, C. R. Congres Geol., 1900, p. 321, et seq.

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back from admitting views which appeared so extreme.  Receptivity is the first virtue of the scientific mind; but, with every desire to lay aside prejudice, many felt unequal to the acceptance of structural features involving a folding of the earth-crust in laps which lay for scores of miles from country to country, and the carriage of mountainous materials from the south of the Alps to the north, leaving them finally as Alpine ranges of ancient sediments reposing on foundations of more recent date.  The historian of the subject will have to relate how some who finally were most active in advancing the new views were at first opposed to them.  In the change of conviction of these eminent geologists we have the strongest proof of the convincing nature of the observations and the reality of the tectonic features upon which the recent views are founded.