VOLCANO OBSERVATIONS. 191 
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. Jf the voleanie 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 contemporaneous 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 under surfaces, 
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 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 recorded. 
In connection with this subject, it should be remembered that very 
many interesting minerals are developed near the junction of igneous 
rocks with those which they traverse. Series of rock-specimens illus- 
trative 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 frag- 
ments of rock which have been ejected from a volcanic vent. These, 
