OF VOLCANIC ROCKS. 29 



Laws relative to the Age of Massive Eruptions. 

 i 



The succession of massive eruptions during the Tertiary and Post-tertiary ages 

 has taken place in the following order : 



1st. Propylite. 



2d. Andesite. 



3d. Trachyte. 



4th. Rhyolite. 



5th. Basalt. 



This singular mode of succession, in which no regularity (as to increase or 

 decrease of silica or specific gravity, or as to a gradual change of mineral composition) 

 can be discovered at first sight, and which might indeed appear to be devoid of order, 

 and to bear the character of such a succession as might have been occasioned by the 

 cooperation of accidental circumstances in one single country, can nevertheless be 

 proved to exist in widely separated parts of the globe. It may justly be objected to 

 this assertion, that observations in regard to the relative age of different volcanic rocks 

 are scarce, and hardly sufficient to establish definitely such a law. But hitherto no 

 deviation from it has been discovered, 10 and it appears to be true for all volcanic regions 

 on the globe, though with this restriction, that the epochs marked in each country by 

 the ejection of rocks of any certain orders have not been contemporaneous in different 

 countries. The commencement of eruptive activity in the Tertiary epoch has been ear- 

 lier at one place than at another ; it is its further mode of development in regard to the 

 nature of the matter ejected which has everywhere been regulated by the same definite 

 relations, though it has been independent, in some measure, at each place, or, to use 

 a more correct expression, over the area of each belt of eruptive activity. We have 

 to mention another restriction. Abrupt passage may be said to be an almost unknown 

 conception in geological matters, where the order in time is concerned ; nor has it to 

 be applied to the order of succession of volcanic rocks. The eruptions of propylite 

 appear not to have been interrupted by the ejection of any other rocks. This may 

 too be said, though less strictly, of the andesitic epoch. But some, as it were, retarded 

 eruptions of andesite, which, however, have always been insignificant, may occasion- 

 ally be traced during the first part of the trachytic epoch, and similar relations exist 

 between trachyte and rhyolite. Basalt, however, appears to have had everywhere its 

 own epoch of ejection, uninterrupted by the massive eruptions of any other rock of 

 the orders just mentioned. 



We proceed to a short review of some salient facts observed in those coun- 

 tries where the mutual relations of volcanic rocks in regard to their age have been 

 made an object of study. 



In Hungary and Transylvania, propylite, as we have had occasion to mention, 

 was ejected first of all volcanic rocks, and may be said to have inaugurated all sub- 



10 It must be borne in mind that we are speaking of massive eruptions. Apparent exceptions are known to occur 

 with rocks ejected from volcanoes, of which mention will be made hereafter. 



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