TRAP OF THE OLD RED SANDSTONE PERIOD. 547 
years in an upright position after they were killed by the 
shower of burning ashes, giving time for a partial decay of 
the interior, so as to afford hollow cylinders into which the 
spores of plants were wafted. These spores germinated and 
grew, until finally their stems were petrified by carbonate 
of lime like some of the remaining portions of the wood of 
the containing Sigillaria. Mr. Carruthers has discovered 
that sometimes the plants which had thus grown and be¬ 
come fossil in the inside of a single trunk belonged to several 
distinct genera. The fact that the tree-bearing deposits now 
dip at an angle of 40° is the more striking, as they must clear¬ 
ly have remained horizontal and undisturbed during a long 
period of intermittent and contemporaneous volcanic action. 
In some of the associated carboniferous shales, ferns and 
calamites occur, and all the phenomena of the successive 
buried forests remind us of the sections (pp. 410, 411) of the 
N^ova Scotia coal-measures, with this difference only, that in 
the case of the South Joggins the fossilization of the trees 
was effected without the eruption of volcanic matter. 
Trap of the Old Red Sandstone Period. —By referring to the 
section explanatory of the structure of Forfarshire, already 
given (p. 74), the reader will perceive that beds of conglom¬ 
erate, No. 3, occur in the middle of the Old Red Sandstone 
system, 1, 2, 3, 4. The pebbles in these conglomerates are 
sometimes composed of granitic and quartzose rocks, some¬ 
times exclusively of different varieties of trap, which last, al¬ 
though purposely omitted in the section referred to, is often 
found either intruding itself in amorphous masses and dikes 
into the old fossiliferous tilestones. No. 4, or alternating with 
them in conformable beds. All the different divisions of the 
red sandstone, 1, 2, 3, 4, are occasionally intersected by dikes, 
but they are very rare in Nos. 1 and 2, the upper members of 
the group consisting of red shale and red, sandstone. These 
phenomena, which occur at the foot of the Grampians, are 
repeated in the Sidlaw Hills; and it appears that in this 
part of Scotland volcanic eruptions were most frequent in 
the earlier part of the Old Red Sandstone period. The trap- 
rocks alluded to consist chiefly of feldspathic porphyry and 
amygdaloid, the kernels of the latter being sometimes calca¬ 
reous, often chalcedonic, and forming beautiful agates. We 
meet also with claystone, greenstone, compact feldspar, and 
tuff. Some of these rocks look as if they had flowed as 
lavas over the bottom of the sea, and enveloped quartz peb¬ 
bles which were lying there, so as to form conglomerates 
with a base of greenstone, as is seen in Lumley Den, in the 
Sidlaw Hills. On either side of the axis of this chain of hills 
