336 SIK A. GEIKIE ON THE TEET1ABY [May 1 896, 



in some degree to a flowing movement of the nearly solidified rock. 

 There is, however, more than this merely structural difference 

 between the several bands. They differ to some extent in the 

 relative proportions of the minerals, especially of olivine and augite ; 

 which points to a considerable flowing movement at an early stage 

 in a magma which was initially not homogeneous.' 



Where the banding arises from the distribution of the vesicles, some- 

 what similar weathered surfaces are produced. In some instances, 

 while the basalt is throughout finely cellular, interposed parallel 

 bands of harder, rather finer-grained and less thoroughly vesicular 

 characters serve to give the stratified appearance. Instances may be 

 observed where the vesicles have been crowded together in certain 

 bands, which consequently weather oat differently from the layers 

 above and below them. An excellent illustration of this arrange- 

 ment occurs in the lowest lava but one of the largest of the three 

 picturesque stacks known as Macleod's Maidens, on the western coast 

 of Skye. This lava is thoroughly amygdaloidal, but the vesicles are 

 specially crowded together in certain parallel bands from 1 to 3 

 or 4 inches thick. Some of 



these layers lie close to each Fig. 2. — Banded amygdaloidal basalt 

 other, while elsewhere there showing layers of elongated and 



may be a band of more close- steeply inclined vesicles. Macleod's 



grained, less vesicular material Maidens, Skye. 



between them. But the most 



singular feature of the rock ^Y^^?^^^^^-^** 

 is to be seen in the shape and *..*• --"•Vr*i^'."*" : "l '"5* ~" ••Vv.^v\r^« 

 position of the vesicles that 

 are crowded together in the 

 cellular bands. Instead of 

 being drawn out into flattened ^>*" v£i>" : V ""„ V-" «"l - J.*^ "?:•£?,* 

 forms in the general direction V * .<*.$» "■•»'" * " Vv: •-. **: : ;'>;'' •^** 

 of banding, they are placed ^^t\^<K^\^S^Ou 



together at high angles. Each * * VA ^ VY* WV\V ^MW 



layer remains parallel to the 



general bedding, but its vesicles are steeply inclined in one direction, 

 which doubtless indicates the flow of the still unconsolidated lava. 

 Weathering along these bands, the lava might easily be mistaken 

 at a little distance for a tuff or other stratified intercalation. 1 



Eanded lavas possessing the characters now described are of 

 frequent occurrence among the Inner Hebrides. Many striking 

 examples of them may be seen along the western coast of Skye. Still 

 more abundant in Faroe, they form one of the most conspicuous 

 features in the geology of that group of islands. Along the whole 

 of its western seaboard, on island after island, they are particularly 

 prominent in the lower parts of the precipices, while the upper parts 

 consist largely of amorphous or prismatic sheets. So much do 

 they resemble stratified rocks that it was not until I had landed at 



1 This elongation of vesicles more or less perpendicular to the general 

 bedding may be noticed sometimes even in sills, as will be shown farther on. 



