60 J". W. Judd'-On Volcanos. 



cinder cones ("puys") and lava currents (''chieres"), and that 

 these have been removed by denuding forces ? 



Of the larger volcanic mountains much more striking and con- 

 spicuous evidences will of course remain, in hills of intrusive rock, 

 and more or less isolated plateaux composed of lavas and alternating 

 volcanic agglomerates. Such may be witnessed in the three great 

 ruined volcanos of the Auvergne, those of the Siebengebirge, the 

 Westerwald, the Habichtswald, the Vogelsberg, the Rhongebirge, in 

 Central Germany, the Duppauer Gebirge and the Leitmeritzer Gebirge 

 in Bohemia, and many similar masses in Hungary. Nowhere, how- 

 ever, can the phenomena presented by such a great ruined volcano be 

 better studied than in the interesting Euganean Hills in the North 

 of Italy. Here, as Prof. Suess of Vienna has so well shown, denuda- 

 tion has operated to such an extent as to bring to light in the most 

 admirable manner the general ground-plan of a volcano, — which he 

 believes must have rivalled Etna in dimensions, — besides revealing 

 innumerable instructive sections of its mass. From the centre of 

 this ruined volcanic pile numerous dyke-like masses of vast propor- 

 tions, and composed of different varieties of lava, are found penetrat- 

 ing the beds of tuff in all directions ; these of course marking the 

 several fissures which were formed on the flanks of the cone, and 

 injected with lavas during its gradual growth. 



By examining the relation of such masses of rock, whether of larger 

 or smaller dimensions, the volcanic origin of which has now been 

 placed beyond dispute, to the various sedimentary and fossiliferous 

 formation, they can be proved to have been erupted during every 

 one of the epochs into which geologists have divided the vast periods 

 concerning which the '' records of the rocks " afford us any 

 information. 



The task of determining the geological age of a series of volcanic 

 rocks is often, however, one of considerable difficulty ; and hence 

 most geologists, in constructing their maps, are content with merely 

 indicating the extent and character of the igneous masses, and of 

 perhaps showing also their relations to ih.Q sedimentary deposits, 

 without attempting to classify them, as they do the latter, according 

 to the period of their formation. 



But it is evident that, as we make progress in our study of the 

 earth's crust, we must not be satisfied until we can refer each of the 

 outbursts of volcanic forces to its proper place in the geological his- 

 tory of the district; nor until our maps indicate the ages of the 

 igneous as well as those of the aqueous deposits. Then only indeed 

 will they truly deserve the title of '' geological maps," and we shall 

 be in a position to reason from them concerning the sequence and 

 relations of geological events— whether of the slower movements 

 which affected the earth's crust, during the quiet accumulation of 

 sediments, or of the more sudden and violent ones, that heralded, 

 accompanied and followed the paroxysmal outbursts of tlie volcanic 

 forces. 



It will be useful then to inquire what are the means which the 

 geologist possesses for determining the date of formation of different 

 igneous deposits. 



