J 6 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



later! in the vast pile of altered sediments, there lie numerous bands 

 of undoubtedly igneous and probably volcanic rocks. Over wide 

 tracts of the central and south-western Highlands and of Donegal, 

 some of the most marked and persistent rocks are sheets of diorite, 

 epidiorite, and hornblende-schist, wbich were erupted as molten 

 materials, not improbably as varieties of diabase-lava. Most of 

 these sheets are doubtless intrusive " sills," for they can be 

 observed to break across from one horizon to another. But some of 

 them may possibly be contemporaneous lava-streams. A sheet may 

 sometimes be traced for many miles, occupying the same strati- 

 graphical platform. Such hornblendic rocks occur on a number of 

 horizons between the great band of Ben-Yoirlich grits and the Ben-y- 

 Glo quartzite. One of the most marked of these is a sill, sometimes 

 200 feet thick, which underlies the Loch-Tay limestone. In Argyll- 

 shire also an abundant series of sheets of epidiorite, amphibolite, and 

 hornblende-schist runs with the prevalent strike of the schists, grits, 

 and limestones of that district ; while similar rocks reappear in a 

 like position in Donegal. The frequency of the association of these 

 eruptive rocks with the limestones is worthy of remark. 



Besides the sheets there occur also bosses of similar material, 

 which in their form and their obvious relation to the sheets recall 

 the structure of volcanic vents. They consist of hornblendic rocks, 

 like the sheets, but usually tolerably massive, with much less trace 

 of superinduced foliation. 



But besides the obviously eruptive masses there is another 

 abundant group of rocks which I believe furnishes important 

 evidence as to contemporaneous volcanic action during the accumu- 

 lation of the Dalradian series. Throughout the central and south- 

 western Highlands certain zones of "green schist" have long 

 arrested the attention of the officers of the Geological Survey. They 

 occur more especially on two horizons (Nos. 7 and 9 of the foregoing 

 table), but the peculiar greenish tint and corresponding mineral con- 

 stituents are likewise found diffused through higher parts of the 

 series. So much do they vary in structure and composition that no 

 single definition of them is always applicable. At one extreme are 

 dull green chlorite-schists, passing into a " potstone," which, like 

 that of Trondbjem, can be cut into blocks for architectural pur- 

 poses *. At the other extreme lie grits and quartzites, with a slight 

 admixture of the same greenish-coloured constituent. Between these 



* From such a rock, which crosses the upper part of Loch Fvne, the Duke 

 of Argyll's residence at Inveraray has been built. 



