86 JUDD. — VOLCANOES. 



rock specimens illustrative of a gradual change in characters will 

 be very valuable. 



E. If masses of tuffs and volcanic agglomerates be met with, 

 they will frequently be found to contain crystals (more or less 

 perfect) of various volcanic minerals. Not unfrequently they also 

 yield fragments of rock which have been ejected from a volcanic 

 vent. These, if of aqueous origin, may be searched for fossils ; in 

 all cases, however, they frequently exhibit signs of having under- 

 gone changes by the action of heat, acid vapours, &c. upon them. 

 Such masses should be broken up and carefully examined, for they 

 frequently enclose in their cavities some of the most beautifully 

 crystallised varieties known to the mineralogist. 



The general instructions as to the instruments best adapted for 

 the purpose of the geological observer, and of the tools used for 

 obtaining rock specimens and minerals, are of course equally appli- 

 cable to the student of vulcanology. But as igneous rocks are in 

 many cases especially liable to change by weathering, the greatest 

 efforts should be made to obtain specimens as fresh and little 

 altered as possible. In those cases, however, where the rock 

 assumes any peculiar features in consequence of meteoric actions 

 upon it, specimens both of the unaltered and of the altered rock 

 are desirable. 



The work of reference which will be found most serviceable to 

 the traveller who may come across volcanic districts is Mr. G. 

 Poulett Scrope's " Volcanos " (second edition, revised and enlarged, 

 1872), published by Longman and Co. In this work detailed 

 descriptions of the interesting phenomena of volcanic action are 

 given, and ample illustrations of most of the points adverted to in 

 these notes will be found. 



