ANNIVERSARY ADDRESS OF THE PRESIDENT. 15I 



This volcanic ring runs as a tract of higher ground encircling 

 the hollow in which the Permian red sandstones lie, and forming a 

 marked chain of heights above the Carboniferous country around. 

 It is built up of a succession of beds of different lavas, with occa- 

 sional ashy partings, which present their escarpments towards the 

 coal-field outside, and dip gently into the basin under the inner 

 trough of brick-red sandstones. The lavas are dull reddish or 

 purplish-grey to brown or almost black rocks ; sometimes compact 

 and porphyritic, but more usually strongly amygdaloidal, the vesicles 

 having been filled up with calcite, zeolites, or other infiltration. 

 The porphyritic minerals are in large measure dull red earthy 

 pseudomorphs of haematite, in many cases after olivine. These 

 rocks have not yet been studied in regard to their composition and 

 microscopic structure. A few slides, prepared from specimens 

 collected in Ayrshire and Mthsdale, have been examined by Dr. 

 Hatch, who finds them to present remarkably basic characters. 

 One from Mauchline Hill is a picrite, composed chiefly of olivine 

 and augite, with a little striped felspar. Others from the Thornhill 

 basin in Dumfriesshire show an absence of olivine, and sometimes 

 even of augite. The rock of Morton Castle consists of large 

 crystals of augite and numerous grains of magnetite in a felspathic 

 groundmass full of magnetite. Around Thornhill are magnetite- 

 felspar rocks, composed sometimes of granular magnetite with 

 interstitial felspar. Throughout all the rocks there has been a 

 prevalent oxidation of the magnetite, with a consequent reddening of 

 the masses. 



That these are true lava-flows, and not intrusive sills, is suffi- 

 ciently obvious from their general outward lithological aspect, some 

 of them being essentially sheets of slag and scoriae. Their upper 

 surfaces may be found with a fine indurated red sand wrapping 

 round the scoriform lumps and protuberances, and filling in the 

 rents and interspaces, as in the case of the Old Eed Sandstone 

 lavas already referred to. Further evidence to the same efi'ect is 

 supplied by the fragmentary materials accompanying them. Here 

 and there, under the platform of bedded lavas, lie bands of brick-red 

 sandstone, full of fragments of slag and fine volcanic dust. Similar 

 materials may be observed to form partings between some of the 

 successive flows. But the most abundant accumulation of such 

 detritus is to be seen immediately above the zone of lavas, where it 

 contains the records of the closing phases of volcanic activit}^ in the 

 south-west of Scotland during the Permian period. Thick beds of tuff 



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