228 
THE TERTIARY VOLCANOES 
BOOK VIII 
derived partly from eruptions of tine dust, partly from subaerial disintegra- 
tion of the basalt-sheets. Some layers ol these finer strata are full of 
remains of much macerated plants. 
Other thin coaly intercalations have been observed among the basalts of 
Canna, some of which may possibly mark still higher horizons than those 
now described. But, confining our attention to the regular sequence of 
intercalations exposed along the Sunday coast, we find at least four distinct 
platforms of interstratified sediment among the plateau-basalts of this 
district. Each of these marks a longer or shorter interval in the outflow 
of lava, and points to the action of moving-water over the surface of the 
lava-fields. 
We may now consider the probable conditions under which this inter- 
vention of aqueous action took place. The idea that the sea had anything 
to do with these conglomerates, sandstones, and shales may be summarily 
dismissed from consideration. The evidence that the basalt-eruptions took 
place on a terrestrial surface is entirely convincing, and geologists are now 
agreed upon this question. 
Excluding marine action, we have to choose among forms of fresh water 
between lakes on the one hand and rivers on the other. That the 
agency concerned in the transport and deposition of these strata was that 
of a river may be confidently concluded on the following grounds : — 
1. The large size and rolled shape of the boulders in the conglomerates. 
To move blocks several tons in weight, and not only to move them but to 
wear them into more or less rounded forms, must have required the opera- 
tion of strong currents of water. The coarse detritus intercalated among the 
basalts is quite comparable to the shingle of a modern river, which descends 
with rapidity and in ample volume from a range of hills. 
2. The evidence that the materials of the conglomerates are not entirely 
local, but include a marked proportion of foreign stones. The proofs of 
transport are admirably exhibited by pieces of Torridon Sandstone, epidotic 
grit, quartzite, and other hard rocks none of which occur in situ except at 
some distance from Ganna. These stones are often not merely rounded, but 
so well smoothed and polished as to show that they must have been rolled 
along for some considerable time in water. 
3. The lenticular character and rapid lithological variations of the 
strata, botli laterally and vertically. The coarse conglomerates die out as 
they are followed along their outcrop and pass into finer sediment. They 
seem to occur in irregular banks, which may not be more than 200 feet 
broad, like the shingle-banks of a river. The coarser sediment generally lies 
in the lower part of the sedimentary group. But cases may be observed, 
such as that shown in Fig. 269, where fine sediment, laid down upon the 
bottom conglomerate, has subsequently been overspread by another inroad of 
coarse shingle. Such alternations are not difficult to understand if they are 
looked upon as indicating the successive floods and quieter intervals of a 
river. 
For these reasons I regard the platforms of sedimentary materials inter- 
