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SIK A. GEIKIE ON THE TEETIAEY 



[May 1896, 



Fig. 10. — Section of wall 

 of another neck of agglo- 

 merate in the same group 

 with those represented in 

 PI. XV. 



In the fragment of geological history so picturesquely laid bare 

 on the Stromo cliffs we are presented with a significant illustration 

 of what seems to have been a common type of volcanic vent in 

 the Tertiary basalt-plateaux. By the 

 fortunate accident that denudation has 

 not proceeded too far, we are able to 

 observe the original tops of at least two 

 of the vents, and to see how such 

 volcanic orifices, which were doubt- 

 less abundant all over these plateaux, 

 came to be entombed under the ever- 

 increasing pile of accumulating basalt. 



There is still one feature of interest 

 in these cliff-sections which deserves 

 notice here. Every geologist who has 

 studied the composition of the basalt- 

 plateaux has remarked the compara- 

 tively insignificant part played by 

 tuffs in these volcanic accumulations. 

 Hundreds of feet of successive basalt- 

 sheets may often be examined without 

 the discovery of any intercalation of 

 fragmental materials, and even where 

 such intercalations do occur they are 



for the most part quite thin and extremely local. I found it 

 impossible to scale the precipice for the purpose of ascertaining 

 whether around the Stromo vents, and connected with them, there 

 might not be some beds of tuff inters tratified between the basalts. 

 If such beds exist, they can be of only trifling thickness and extent. 

 Here, then, are examples of once active vents, the funnels of which 

 are still choked up with coarse fragmentary ejections, yet from 

 which little or no discharge of ashes and stones took place over the 

 surrounding ground. They seem to have been left as maare or 

 crater-like hollows on the surface of the lava-fields. 



The next example of a neck which I will describe occurs on the 

 cliffs that form the southern side of the sheltered inlet known as 

 Portree Bay in the Isle of Skye. These cliffs, the seaward escarpment 

 of the basalt-plateau, rise above a platform of Jurassic sandstones 

 and shales. At Camas Garbh, where they have been trenched by 

 a small rivulet, aided by the presence of two dykes, the gully thus 

 formed exposes a section of a neck of agglomerate that underlies 

 the basalts of the upper half of the cliff. This neck is connected 

 with a thick deposit of volcanic conglomerate and tuff which, lying 

 between the basalts, extends from the neck to a considerable 

 distance. The general relations of the rocks in this cliff are 

 represented in fig. 11. 



The agglomerate (b in fig. 11) is quite tumultuous, and here 

 and there strikingly coarse. Some of its included blocks measure 

 5 feet in length. These fragments represent most of the varieties 

 of the lavas of the district. Large slaggy masses are abundant, 

 and sometimes exhibit the annelid-like elongation of the vesicles 



