7 8 PROCEEDIN^GS OF THE GEOLOGICAL SOCIETY. 



essential characters the Lewisian rocks of the north-west of Scotland. 

 This ancient ridge, however, is flanked by the local base of the 

 Dalradian series, consisting of a group of coarse volcanic agglome- 

 rates, tuffs, and lavas, which are succeeded by and pass up into green 

 chloritic schists and silvery mica-schists like those of the Scottish 

 Highlands. The importance of the discovery of so distinct a vol- 

 canic group in rocks of such antiquity led me recently to pay a 

 second visit to the district, and to follow the volcanic zone for some 

 miles to the south-west. 



The lavas are chiefly dull greenish, fine-grained, sometimes highly 

 amygdaloidal, the amygdules consisting generally of calcite. Close 

 to the gneiss these rocks are not foliated, but they show abundant 

 lines of shearing, along which they assume a schistose structure. 

 The cores of solid uncrushed rock still retain perfectly the original 

 spherical form of the vesicles. Under the microscope they are found 

 to be diabases, in which the lath-shaped felspars, and the augite which 

 these penetrate, are sometimes tolerably fresh, while in other parts 

 fibrous chlorite, granular epidote, and veins of calcite bear witness 

 to the metamorphism which they have undergone *. 



There occur also, but less abundantly in the part of the district 

 examined by me, platy felsitic rocks which have suff'ered much from 

 shearing, and consequently have acquired a fissile slaty structure. 



The agglomerates are made up of angular, subangular, and rounded 

 fragments of different lavas embedded in a matrix of similar compo- 

 sition. This matrix has in places become quite schistose, and then 

 closely resembles some parts of the "green schists" already referred 

 to. Of the embedded stones the great majority consist of various 

 felsites which, weathering with a thick white opaque crust, are inter- 

 nally close-grained dull grey or even black, sometimes showing flow- 

 structure, and of all sizes up to 8 inches in diameter or more. 

 There are also fragments of the basic lavas. On many of the rooky 

 hummocks no distinct bedding can be made out in the agglomerate, 

 but in others the rock is tolerably well stratified. 



The tuffs are fine silky schistose rocks, and seem to have been 

 largely derived from basic lavas. They have suffered more than 

 any of the other rocks from mechanical deformation, for they pass 



* In the preparation of this Address I have had a large collection of thin 

 slices cut from the more important rocks which I have myself collected, and 

 to which I have occasion to refer in these pages. My colleague, Dr. Hatch, has 

 been good enough to make a preliminary examination of this collection for me, 

 and has supplied me with the notes from which the statements in the Address 

 regarding the microscopic structure of the rocks are mainly taken. 



