ANCIENT VOLCANIC ROCKS. 



I^HE continental penologists, with few exceptions, con- 

 tinue to make a fundamental distinction between the 

 "older" {i.e., pre-Tertiary) volcanic rocks and the "younger" 

 (Tertiary and Recent). The hold which this doctrine has 

 maintained in almost every country, but our own, depends 

 upon a conjunction of causes. One of these causes is the 

 almost complete absence in the European area of volcanic 

 activity during Mesozoic times. Owing to this long hiatus, 

 the fresh Tertiary lavas come to be, as a whole, strongly 

 contrasted with their much older Palaeozoic equivalents, 

 which in course of time have usually suffered much from 

 the secondary alterations to which volcanic products are 

 peculiarly liable. By regarding the accidental characters 

 thus induced as essential, an impression of wide difference 

 between the older and the newer lavas is fostered. 



Another curious fact has contributed to confirm the 

 same idea. Germany, France, and Italy form part of a 

 region within which the Tertiary volcanic rocks belong, in 

 great measure, to somewhat peculiar types. Their relative 

 richness in alkalies frequently gives rise to special minerals 

 and associations of minerals ; and the occurrence in other 

 parts of the world of corresponding rocks having a high 

 geological antiquity has been the more easily overlooked 

 since some of these special minerals, such as leucite, are 

 eminently liable to chemical destruction. 



The doctrine of " older " and " younger " volcanic rocks, 

 itself a surviving relic of the theories of Werner, has been 

 fortified by these circumstances, and is incorporated in the 

 petrological classifications and nomenclature current in 

 continental countries. The leaders in this branch of 

 geological science have, it is true, seen reason in some 

 instances to modify their views ; but it is a singular illustra- 

 tion of the conservative spirit that, even while formally 

 abandoning the idea that the essential characters of igneous 

 rocks depend on their geological age, they still retain it in 



