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THE JOURNAL OF GEOLOGY.

rarity in Mesozoic times. It is, however, also connected with the Wernerian doctrine of the non-recurrence of certain physical conditions in the earth's development, as contrasted with the uniformitarianism of Hutton and Lyell. The absence of volcanic types in Europe which serve to bridge over the sharp contrast between those of the Carboniferous and Tertiary, is being rapidly compensated by the discovery of such rocks in other regions. Fortunate finds of even pre-Cambrian lavas so perfectly preserved as to demonstrate their practical identity, both chemically and structurally, with recent products is tending to weaken the old distinction on the continent. There are now many signs of progress toward the idea that the characters regarded as belonging peculiarly to the older effusive rocks are better explained through changes subsequent to their solidification.

Thus Ludwig in 1861,[1] Vogelsang in 1867,[2] and Lossen in 1869,[3] regard some quartz-porphyries as only devitrified glasses, identical with those of modern volcanic regions; Kalkowsky,[4] and recently Sauer[5] and Vogel,[6] have also brought convincing proof that such is often the case.

Gümbel says: "Es scheint in dieser Beziehung denn doch eher gerechtfertigt, zunächst das petrographisch Gleiche auch gleich zu bezeichnen, als in einzelnen Fällen ein neues Princip, das des Alters, in die Petrographie einzuführen, welches bei den meisten übrigen Fällen nicht verglichen und berücksicht werden kann;"[7]

And Rosenbusch also remarks:

"Man hat den geologischen Alter der Eruptivgesteine bisher ein höheres bestimmendes Moment für die structurelle und mineralogische Ausbildung dieser zugeschrieben als demselben in Wirklichkeit zukommt."[8]

  1. ↑ Erl. z. geol. Karte Hessens, Bl. Dieburg, p. 56 1861.
  2. ↑ Philosophie der Geologie, pp. 144-146, 1867.
  3. ↑ Abh. Berl. Ak., 1869, p. 85.
  4. ↑ Tschermak's Min. Mitth, pp. 31 and 58, 1874.
  5. ↑ Erl. zur geol. Specialkarte Sachsens, Bl. Meissen, pp. 81-91, 1889
  6. ↑ Abh. geol. Landesanstalt von Hessen, vol. ii., p. 38, 1892.
  7. ↑ Grundzäge der Geologie, 1888, p. 85.
  8. ↑ Die massigen Gesteine, 2d. ed., 1887, p. 4.
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