24 
VOLCANIC PRODUCTS 
BOOK I 
Under tlie vast compression to which the earth’s crust is subjected during 
terrestrial contraction, the most obdurate rocks are crushed into frag- 
ments varying from large blocks to the huest powder. Tliis comminuted 
mateiial is driven along’ in the direction of thrust, and when it comes to 
rest presents a streakiness, with curving lines of Ilow round the larger 
fragments, closely simulating the structirre ot many rhyolites and obsidians. 
Tt is only by attention to the local surroundings that such deceptive re- 
semblances can be assigned to their true cause. 
4. lire LUSPO.siTioN of lavas in .sheets or eeij.s is the result of 
successive outflows of molten rock. Such sheets may range from only a 
yard or two to several hundred feet in thickness. As a rule, though with 
many exceptions, the basic lavas, such as the basalts, appear in thinner beds 
than the acid forms. This difference is well brought out if we compare, 
for instance, the massive rhyolites or felsites of Xorth Wales with the thin 
sheets of basalt in Antrim and the Inner Hebrides. The regularity of the 
bedded character is likewise more definite among the basic than among 
the acid rocks, and this contrast also is strikingly illustrated by the two 
series of rocks just referred to. The rhyolites and felsites, sometimes also 
Fig. 10.— Lumpy, irregular tracliytie Lava-streams (Carboniferous), East Linton, Haddingtonshire. 
the trachytes and andesites, assume lumpy, irregular forms, and some httle 
care may be recpdred to trace their upper and under surfaces, and to 
ascertain that they really do form continuous sheets, though varyiim much 
in thickness from place to place (Fig. 10). Like modern acid lavas, they 
seem to have flowed out in a pasty condition, and to have been heaped up 
round the vents in the form of domes, or with an irregular hummocky or 
mounded surface. The basalts, and dolerites, and sometimes the andesites, 
have issued in a more fluid condition, and have spread out in sheets of more 
uniform thickness, as may be instructively seen in the sea-cliffs of Antrim, Mull, 
Skye, and the Faroe Islands, where the horizontal or gently-inclined Hows 
of basalt lie upon each other in even parallel beds traceable for considerable 
distances along the face of the precipices (Fig.s. 11, 265, and 286). The ande- 
stes of the Old Eed Sandstone (Figs. 99, 100) and Carboniferous series (Fig.s. 
I Of, 108, 111, 112, 113, 123) m Scotland likewise form terraced hills. 
The length of a lava-stream may vary within wide limits. Sometimes an 
outflow of lava has not reached the base of the cone from the side of which 
it issued, like the obsidian stream on the Hanks of the little cone of the 
island of Volcano. In other cases, the molten rock has flowed for forty or 
fifty miles, like the copious Icelandic lava-floods of 1783. In the ba.salt- 
plateaux of the Inner Hebrides a single sheet may sometimes be traced for 
several miles. 
