VOLCANO OBSERVATIONS, 189 
e. Structure. All peculiarities of jointed, and especially of 
columnar, structures are worthy of being recorded. Note the 
features presented by the upper and lower part of the current, and 
any changes in its course; also if columns be divided by transverse 
joints, and the features presented by these, etc. 
Ff. Chemical and mineralogical constitution. If the appearance 
of the rock does not suggest at once the class to which it belongs, 
and the component minerals cannot be detected with a lens, recourse 
may sometimes be had to a determination (even roughly) of its 
specific gravity. 
g. Sometimes lavas contain large masses of included minerals, 
either crystallised around cavities, or resulting from the alteration of 
ejected blocks. These are very interesting, and should be carefully 
collected. 
h. Cavities, or air-bubbles, in comparatively recent lavas, are 
frequently found coated with beautifully crystallised minerals. And 
when the rocks are of older date, the similar cavities may be lined or 
filled with crystals of zeolites and other minerals. 
B. Nature or BEDS LYING BETWEEN LAVA-CURRENTS. 
These are of the utmost interest and value to the geologist, but 
unfortunately the ordinary mode of weathering of volcanic rocks is 
such as greatly to obscure the interbedded deposits by a talus of 
fallen fragments. The best opportunities for their study are afforded 
by sea-cliffs, and deep ravines or river-gorges, which should therefore 
be carefully examined. In such situations we may expect to find— 
a. Burnt soils (“ Laterites” of Lyell), usually of a brick-red colour, 
and affording various evidences of their modes of origin. 
b. Coal- or Lignite-seams. These are very frequently observed. 
Note if they rest upon an “underclay” (an old soil with rcots), and 
if they contain wood, leaves, or other plant-remains, with recognisable 
structure. 
c. Ash-beds. These are sometimes composed of such impalpable 
dust as to constitute a matrix in which delicate leaves, shells and 
even insect remains are exquisitely preserved. The collection of 
every trace of organic bodies is of the greatest importance. 
d. Stratified tuffs. Note especially the degree and nature of 
their stratification ; also whether they are loose or indurated. They 
may contain shells and plants of terrestrial or marine origin. Record 
the elevations at which the latter are found. 
e. Gravels or other deposits. Note their characters and materials, 
and, if possible, denne their mode of origin. 
