J. W. Judd — On Vokanos. 61 



The loose masses composed of scorige, tuffs, or ashes, whicli are 

 accumulated during a volcanic outburst, are usually among the first 

 of its products that yield to the wasting effects of denudation ; and 

 hence the organic remains, which are not unfrequently overwhelmed 

 by or enveloped in such materials, have in too many instances totally 

 disajipeared. Occasionally, however, in consequence of their being 

 covered up by masses of hard lava, or through being buried under 

 subsequent sedimentary deposits, such fossiliferous volcanic agglo- 

 merates, tuffs or ashes have been preserved, and furnish, us with 

 direct palaeontological evidence of the period of the outbursts by 

 whicb they were formed. Sometimes, indeed, owing to the process 

 of silicification having gone on in the mass, the fossils contained 

 in such igneous rocks are unrivalled for their beauty and admirable 

 preservation. As examples of such we may mention the terrestrial 

 plants, insects, shells and bones of Saros Patak, and other localities 

 in Hungary, of Lipari, Somma and Ardtun ; the lacustrine shells, 

 plants, fishes and mammals of Bohemia and the Auvergne ; and the 

 marine plants, shells and fishes of Ischia and the Yicentine. 



But in too many cases, unfortunately, this direct palseontological 

 evidence of the date of a volcanic outburst is altogether wanting, and 

 the geologist is compelled to fall back upon less direct, though not 

 necessarily less conclusive, evidence. Foremost among such we 

 must mention the mode of association of the igneous rocks in question 

 with sedimentary deposits, the age of which is known. 



Here, however, we must point out that the mere association of 

 igneous masses with a particular geological formation affords no 

 proof whatever of their contemporaneity ; and geologists have 

 not unfrequently been led into the most serious error through too 

 hastily assuming such to be the case. Thus in the West of Scotland 

 the basalts are so constantly associated with the Jurassic rocks, that 

 ever}'- one was at first led astray concerning their age by this acci- 

 dental connexion ; and it was only by the discovery of Miocene 

 plants in their tuffs, and of their superposition to the highest 

 Cretaceous rocks, that the fallacy of the inference as to their Jurassic 

 age was made apparent. So also in Bohemia, as I had an oppor- 

 tunity of observing under the guidance of Prof Fritsch of Prague, 

 many isolated masses of basalt are found in the midst of outliers of 

 Chalk ; in this case, also, the association is purely accidental, the 

 basalts being unquestionably of late Tertiary age, while the masses 

 of Clialk around them appear to owe their preservation to a certain 

 amount of induration produced in them by the igneous intrusion. 



Thus it becomes clear that, in reasoning on the age of igneous 

 rocks, we must convince ourselves not only of the fact, but of the 

 nature of their association with the sedimentary rocks. 



Where igneous masses are found clearly overlying sedimentary 

 deposits, or where they penetrate between the aqueous beds in sheets, 

 producing alteration both in the strata above and below them ; or 

 where they traverse the sedimentary formations in dykes, or enclose 

 portions of the latter, whether as entangled fragments caught up 

 in lava, or as ejected masses enclosed in the agglomerates ; or again 



