JtJDD.— VOLCANOES. §5 



on the look-out for (their vapours often render them conspicuous 

 at great distances), and their phenomena should be carefully re- 

 corded. Specimens of hot and mineral water should be sealed up 

 in bottles, and brought home for examination and analysis. 



g. Deposits of siliceous sinter, travertine, Sfc. These, besides 

 yielding interesting varieties of minerals and illustrations of their 

 mode of formation, often contain incrusted or mineralised remains 

 of plants or animals which may be of great interest. 



(In the event of the observer being so fortunate as actually to 

 witness an eruption of a volcano, every detail that he can supply 

 may be of scientific value. Especially should he note the appear- 

 ances presented by the ascending column of vapour and fragmen- 

 tary materials issuing from the crater, the height to which this 

 rises, the nature, sequence, and rate of the explosions to which it 

 is due, and the sounds which accompany them. All earthquake 

 shocks and tremblings of the ground should of course be recorded. 

 If lava streams are seen flowing, their rate of motion and attendant 

 phenomena should be carefully noted.) 



II. If the volcanic rocks have evidently been subjected to great 

 denudation, the following points should be more particularly 

 attended to : — 



A. The composition, textures, and various structures of the 

 different lavas should be carefully observed, and all zeolites or other 

 minerals in their included cavities collected. 



B. If the igneous rocks be found alternating with sedimentary 

 ones, all fossil remains which can be obtained from the latter will 

 have a double value, as throwing light on the age both of the 

 aqueous and the volcanic rocks. But it will be especially necessary 

 to notice whether the igneous masses be interbedded and contem- 

 poi^aneous with the aqueous deposits, or intrusive and subsequent 

 to them. In seeking to determine this point, it must be borne in 

 mind that, — 



Lava streams have slaggy or scoriaceous upper and imder sur- 

 faces, and that they only alter the rocks upon which they rest. 



Intrusive sheets, on the other hand, are seldom scoriaceous, and 

 alter the rocks both below and also above them. They moreover 

 occasionally cross the lines of bedding of the strata, and send off 

 dykes or veins into them. 



C. If possible, the lavas of the district should be traced up to 

 central masses of intrusive rocks. The forms assumed by these in 

 weathering should be sketched, and specimens illustrating the 

 different characters which they assume and the minerals they con 

 tain be collected. 



D. All the phenomena of metamorphism exhibited by the strati- 

 fied rocks in the vicinity of intrusive masses, whether dykes, sheets, 

 or bosses, should be looked for, and their nature and extent re- 

 corded. In connexion with this subject, it should be remembered 

 that very many interesting minerals are developed near the junc- 

 tion of igneous rocks with those which they traverse. Series of 



