118 PROCEEDINGS OF THE AMERICAN ACADEMY. 



necks. In Fifeshire, eighty of them have been counted in an area 

 measuring eighteen kilometers by ten kilometers. In western Ayrshire, 

 sixty vents are found in an area measuring sixty by thirty kilometers, 

 and, of those vents, twenty are necks occurring within an area of thirty- 

 five square kilometers. In the great majority of cases, Geikie and his 

 collaborators have been unable to find any connection between the 

 positions of the necks and lines of dislocation. The Carboniferous 

 strata have suffered sieve-like perforation like that of the Jurassic beds 

 in Suabia. In each of the Scottish districts, the lower part of the very 

 thick Carboniferous sedimentary series carries numerous thick sills of 

 dolerite. These sills are mapped as chiefly Carboniferous in date, but 

 Geikie thinks that some of the Fifeshire sills at least are Permian. 

 The steady association of tuff-neck and sill in the Scottish shires 

 scarcely looks accidental. 



Gas emanations from the magma forming these actual intrusives or 

 similar ones occurring in the underlying pre-Carboniferous formations, 

 together with the possible emanation of gas from the heated country- 

 rock, would seem to be competent to explain most of the tuff- necks. 

 Explosive drilling (diatremes) and gas-fluxing might in turn dominate 

 in the opening of the vents. The total activity must be small, because 

 each gas-emitting or lava-emitting chamber was small. 



The list of districts where the writer suspects secondary vulcanism 

 includes also the area of necks in Noss Sound, Shetland. ^^ These are 

 small and the volcanic throats are filled with a coarse agglomerate 

 of sandstone and shale. Peach and Home infer that the vents never 

 emitted any streams of lava.^® The eruptivity is referred to the Lower 

 Old Red Sandstone period. The date may be nearly the same as that 

 of the injection or the thick sills and dikes which abound in the Noss 

 Sound region. 



A Necessary Division of Central Vents. — It is obviously difficult to 

 devise field criteria which shall infallibly distinguish central eruptions 

 respectively originating in main abyssal injections and in satelhtic in- 

 jections. Long and strong activity, large outflow of lava, and align- 

 ment in chains will generally characterize the central vents of abyssal 

 injections. Brief activity, small output of lava, cluster grouping, and 

 traces of surface deformation in the region are the expected features 

 of the central vents of satellitic injections. As one or more of these 

 features is absent or is obscured, the classification is hard to apply. 



'^ A. Geikie, Quart. Jour. Geol. Soc. London. Presidential Address, 48, 

 95 (1892). 



" B. N. Peach and J. Home, Trans. Roy. Soc. Edinburgh, 32, 359 (1884), 

 and 28, 418 (1878). 



