364 SIR A. GEIKIE ON THE TERTIARY [May 1 896,. 



merely a few pebbles between the basalts. It lies in a kind of 

 channel or hollow among these lavas, which in an east-and-west 

 direction cannot be more than about 65 yards broad. 



Probably still higher in the series of basalts is another intercala- 

 tion of sedimentary layers which may be seen in the little bay to 

 the east of Tallabric, rather more than a mile to the west of the 

 Creag nam Eaoileann. It rests upon a coarsely slaggy amygdaloid, 

 and is from 6 to 10 feet in thickness. The lower and larger part 

 of the deposit consists of greenish pebbly sandstone and fine con- 

 glomerate, largely composed of basaltic detritus, but including 

 abundant well-smoothed and polished pebbles of Torridon Sandstone, 

 green grit, quartzite, etc. The stones vary from mere pea-like 

 pebbles up to pieces 2 or 3 inches long, the largest being 

 generally fragments of slag and amygdaloid which are less water- 

 worn than the sandstones and other foreign ingredients. The 

 uppermost 2 or 3 feet of the intercalation consist of dark carbon- 

 aceous mudstone or shale, made up in large measure of volcanic 

 detritus, which may have been derived partly from eruptions of 

 fine dust, partly from subaerial disintegration of the basalt-sheets.. 

 Some layers of 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 hori- 

 zons than those now described. But, confining our attention to the 

 regular sequence of intercalations exposed along the Sanday 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 

 intervention 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 shapes of the boulders in the con- 

 glomerates. 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 operation 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- 



