8q proceedings of the geological society. 



tides of detrital felsite of which they are essentially composed. 

 The brecciated varieties exhibit finely-streaked flow-structure in 

 some of the felsite fragments. Pieces of andesite, grains of quartz, 

 and other extraneous ingredients appear in the rocks towards the 

 southern limits of their area, where they are associated with and 

 pass laterally and vertically into ordinary non-volcanic sedimentary 

 strata. 



III. Steucttjre of the Ground. — We have now to consider the 

 manner in which these various volcanic products have been grouped 

 around and within the orifices of discharge. The first feature to 

 arrest the eye of a trained geologist who approaches them as they 

 are displayed in one of the ranges of hills in Central Scotland is the 

 banded aspect of the rocks. If, for example, he looks eastward 

 from the head of the Pirth of Tay, he marks on the right hand, 

 running for many miles through the county of Fife, a succession 

 of parallel escarpments, of which the steep fronts face northwards, 

 while their long dip-slopes descend towards the south. On the 

 left hand a similar but higher series of escarpments, stretching 

 far eastwards into Forfarshire, through the chain of the Sidlaw 

 Hills, repeats the same features, but in opposite directions. If he 

 stands on the alluvial plain of the Forth, near Stirling, and looks 

 towards the north, he can trace bar after bar of brown rock 

 and grassy slope rising from base to summit of the western 

 end of the Ochil Hills. Or, if from any height on the southern 

 outskirts of the city of Edinburgh, he lets his eye range along 

 the north-western front of the chain of the Pentland HiUs, 

 especially towards evening, he can follow the same parallel banding 

 as a conspicuous feature on each successive hill that mounts above 

 the plain. 



1. Bedded Lavas and Tuffs. — On a nearer inspection this domi- 

 nant topographical feature is found to correspond with a well- 

 marked stratification of the whole volcanic series. Where two 

 sheets of porphyrite are separated by layers of tuff, sandstone, or 

 conglomerate, a well-marked hollow will generally be found to 

 indicate the junction-line ; but even where the lavas follow each 

 other without such interstratifications, their diff'erences of texture 

 and consequent variations in mode and amount of weathering 

 usually suffice to mark them off from each other and to indicate 

 their trend along the surface. 



It is in the picturesque and instructive coast-sections, however, 

 that the details of this bedded structure are most clearly displayed. 



