148 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The best sections are those exposed along the coast to the north 

 and sonth of Ballantrae. When that ground was first examined 

 by the Geological Survey the hypothetical views in regard to meta- 

 morphism already referred to were in full ascendant, and the rocks 

 were mapped on the same general principles as those which had been 

 followed in Wales. Professor Bonney, however, a few years later 

 recognized the true igneous nature of many of the rocks. He found 

 that there were porphyrite lavas and agglomerates which he regarded 

 as of Old Eed Sandstone age, likewise intrusive serpentines and 

 gabbros *. 



The volcanic materials have not yet been systematically studied. 

 I may say, however, that they include amygdaloidal lavas of the 

 "porphyrite" type, diabases, agglomerates made up of blocks of 

 these lavas, and tuffs, chiefly basic, but including some that are 

 probably felsitic. In Ayrshire massive sheets of amygdaloidal and 

 porphyritic lavas occur which singularly resemble the porphyrites 

 of the Old Eed Sandstone of the same country. Similar rocks 

 appear where the volcanic group is brought to the surface all along 

 the northern part of the Silurian ground ; some of them are 

 markedly slaggy, even with a ropy surface, while the fragmental 

 beds associated with them sometimes consist of blocks of similar slag. 



Along the south-eastern parts of the volcanic area the ejected 

 materials have a more acid character. They consist largely of fine 

 tuffs (probably in great measure felsitic), which towards the north- 

 east are gradually interleaved with ordinary sediment till the ashy 

 character disappears. But, as I have already remarked, there is 

 reason to believe that the overlying greywackes and shales derived 

 part of their material either directly from volcanic explosions or 

 from the attrition of banks of lavas and tuffs exposed to denudation. 



The dying out of the volcanic material towards the north-east 

 probably indicates that the vents of the period lay rather in the 

 central or south-western parts of the district. Unfortunately, the 

 limited extent of the exposures of the rocks makes it a hopeless 

 task to search for traces of these vents over by far the largest part 

 of the area. There are two localities, however, where the search 

 may be made with better prospect of success. One of these is a 

 tract to the north of Sanquhar in Nithsdale. Besides the porphy- 

 rites, which resemble some of the Ayrshire rocks, there occur nume- 

 rous varieties of tuff, which Dr. Hatch has found to present under 

 the microscope many points of interest. Some of them are com- 



* Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 769. 



