i8 
VOLCANIC PRODUCTS 
BOOK I 
Where, for example, a scoriaceous lava is covered with stratified deposits 
which contain pieces of that lava, we may be confident that the rock is an 
interstratified or contemporaneous sheet. It has been erupted after the 
deposition of the strata on which it rests, and before that of the strata 
which cover it and contain pieces of it. In such a case, the geological date 
of the eruption could be precisely defined. Illustrations of this reasoning 
will be given in Chapter iv., and in the account of the volcanic series 
of Carboniferous age in Central Scotland, where a basic lava can some- 
times be proved to be a true How and not an intrusive sill by the fact that 
portions of its upper slaggy surface are enclosed in overlying sandstone, 
shale or limestone. 
2. The presence of glass, or of some result of the devitrification of an 
original glass, is an indication that the rock which exhibits it has once been 
in a state of fusion. Even where no trace of the original vitreous condition 
may remain, stages in its devitrification, that is, in its conversion into a 
stony or lithoid condition, may be traceable. Thus what are called spheru- 
litic and perlitic structures (which will be immediately described), either 
visible to the naked eye or only observable with the aid of the microscope, 
afford evidence of the consolidation and conversion of a glassy into a lithoid 
substance. 
Striking evidence of the foimer glassy, and therefore molten, condition 
of many rocks now lithoid is to be gained by the examination of thin 
slices of them under the microscope. Not oidy are vestiges of the original 
glass recognizable, but the whole progress of tlevitrification may be followed 
into a crystalline structure. The primitive crystallites or microlites of 
different minerals may be seen to have grouped themselves together into 
more or less perfect crystals, while scattered crystals of earlier consolidation 
have been partially dissolved in and corroded by the molten glass. These 
and other characteristics of once fused rocks have to a considerable extent 
been imitated artificially by MM. Eouqmi and Michel Levy, who have fused 
the constituent minerals in the proper proportions. 
Since traces of glass or of its representative devitrified stnictures are so 
abundantly discoverable in lavas, we may infer tlie original condition of most 
lavas to have been vitreou.s. Where, for instance, the outer selvages of a 
basic dyke or sill are coated with a layer of black glass which rapidly passes 
into a fine-grained crystalline basalt, and then again into a more largely 
crystalline or doleritic texture in the centre, there can be no hesitation in 
believing that glassy coating to be due to the sudden chilling and consolida- 
tion of the lava injected between the cool rocks that enclose it. The part 
that solidified first may be regarded as probably representing the condition 
of the whole body of lava at the time of intrusion. The lithoid or crystal- 
line portion between the two vitreous outer layers shows the condition 
which the molten rock finally assumed as it cooled more slowly. 
Some lavas, such as obsidians and pitchstones, have consolidated in the 
glassy form. More usually, however, a lithoid structure has been developed, 
the original glass being only discoverable by the microscope, and often not 
