1871.] 



GEIKIE XERTIABY VOICANIC EOCKS. 



291 



brown ; and their continuity is still further indicated by the slender 

 lines of bright herbage which have taken root along the decaying 

 upper or under surfaces of the flows. Yet, on closer examination, 

 we find them not un frequently to die out, the place of one bed being 

 taken by another, or even by more than one, in continuation of the 

 same horizon. This is particularly noticeable along the cliff-line 

 on the east side of Beinn Bhuidh. There is considerable diversity 

 in the colour and texture, as well as the structure, of the different 

 beds. Some of them, in which the rock is more compact and 

 weathered, are divided by vertical joints, which in some cases in- 

 crease in number tUl the rock acquires a rudely columnar structure. 

 This may be admirably seen along the coast north of the harbour, 

 where a long line of columnar cliff shows in some places curved and 

 radiating columns. Other beds are formed of a dark compact amor- 

 phous mass, usually amygdaloidal, and occasionally very markedly 

 so. A not infrequent variety occurs in the form of a duU green 

 amygdaloidal and scoriaceous rock, in which balls of more compact 

 material are wrapped, as it were, in a softer decomposed base. At 

 the south end of the island, a peculiar band of rock occurs, in which 

 the process of weathering reveals a succession of layers, a few inches 

 thick, formed of nodular pieces of compact blue anamesite or basalt, 

 with a bright red crust. These layers lie a few inches apart, in a 

 soft, dirty-green, crumbling, and often highly amygdaloidal rock. 

 The baud in which these features are seen runs as an intercalation, 

 about 3 or 4 yards thick, among the sheets of hard crystalline 

 anamesite. 



As an illustration of the bedded arrangement of these rocks, and 

 of the way in which they succeed each other along the same hori- 

 zontal plane, reference may be made to the accompanying diagram 

 (fig. 3) of part of the cliif-section north of Kildonan, on the east 

 side of the island. 



Fig. 3. Diagram of interbedded Volcanic RocTcs on the east side of 

 Island of Eigg. 



g. Compact jointed dolerite. /. Dull dirty-green decomposing amyda]oidal 

 dolerite. e. Compact crystalKne dolerite, more finely jointed than bed g. 

 d. Pale grey porphyrite. c. Dolerite, which a little further north is formed of 

 several beds. b. Columnar dolerite. a. Oolitic strata. 



The tests by which the true interbedded or contemporaneous 

 character of the flows of the doleritic plateau can be determined are 

 well exposed in Eigg. 1st. The upper and under surfaces of the 

 successive flows have very commonly a rough slaggy character, even 

 when the central portion is compact and crystalline. In this respect 

 they perfectly resemble sections of recent lava-streams, such, 

 for example, as those exposed along the Bay of Naples, around Torre 



