Yol. 52.] BASALT-PLATE A TJX OF NORTH-WESTERN EUROPE. 



367 



some of the remarkably coarse masses of unstratified conglomerate 

 in Canna Harbour consist largely of ejected blocks from the adjacent 

 vent. 



The only instance which I have observed of erosion of the basalts 

 contemporaneous with the operations of the river that spread out 

 this conglomerate is to be found in the striking stack of Dun Bcag 

 already alluded to. 1 This extraordinary monument of geological 

 history forms an outlying obelisk which rises from the platform of 

 the shore to a height of about 70 feet. Seen from the south-west 

 it appears to consist entirely of bedded basalt resting on some 

 stratified tuff and shale which intervene between these lavas and 

 that of the broad platform of basalt on which the obelisk stands. 



Fig. 18. — Section of the eastern front of the Dun Beag. 



/ 



a. Very slaggy araygdaloidal basalt. 



b. Shales and tuff. 



c. Slaggy and jointed basalts. 



The dotted lines indicate the supposed form of the ravine. 



d. Conglomerate. 



e. Prismatic basalt. 



On that side it presents no essential difference from the structure 

 of the Dun Mor on the west, save that the lower conglomerate of 

 that outlier is here represented by fine sediment, and the upper 

 conglomerate is wanting. The general aspect of this south-western 



1 This pinnacle of rock is referred to by Macculloch in his account of Canna. 

 and is figured in pi. xix. fig. 3 in his work already cited. But neither his 

 description nor his drawing conveys any idea of the real structure of the rock. 



