244 Geikie — Permian Volcanos in Scotland. 



The Permian basin of the Ayr is completely encircled by a ring of 

 trappean rocks, from half a mile to a mile broad, and rising for the 

 most part as a conspicuons ridge, separating the brick-red Permian 

 sandstones inside, from the dull red or purple Carboniferous strata 

 outside. It is the nature of this girdling ring which falls to be con- 

 sidered in the present paper. For the sake of clearness and concise- 

 ness, it may be of advantage to show — 1st, that the ring consists of 

 lava-flows and ashes, and is therefore truly volcanic ; and 2nd, that 

 this volcanic series is a part of the Permian formation. 



I. The accompanying section (Fig. 1), drawn through the village 

 of Mauchline along the north-eastern margin of the basin, shows 

 the general arrangement of the rocks of the district. The ring of 

 igneous rocks rises on the north-west side into the ridge, on which 

 Bums' farm of Mossgiel lies, and reappears again on the sou.th-west 

 side in the cliffs and ravines of Ballochmyle. It consists of suc- 

 cessive sheets of trap, occasionally interstratified with sandstone and 

 sandy ash, and sometimes lying on, sometimes covered by, ash. 

 These trap-sheets have a prevailing dull red, purplish red, or dark 

 brown hue, and are made up of a mixture of labradorite and specidar- 

 iron, with sometimes a little augite. They would be called melapliyres 

 in Germany, and as owe English nomenclature of igneous rocks is so 

 poor, I have fomid it useful to adopt that term. Their mineralogical 

 structure and changes are full of interest, but need not detain us at 

 present. The melaphyres are bedded masses, now compact and 

 poiphyritic, and then passing into a coarse slaggy amygdaloid. 

 Between the beds thin layers of more or less ashy brick-red sand- 

 stone are often found, and it sometimes happens, as on the east side 

 of Mauchline Hill, that perpendicular veins of horizontally stratified 

 red sandstone reticulate the upper part of an amygdaloidal bed. In 

 such cases it is clear that these veins represent old cracks in the lava- 

 flow into which sand was washed before the eruption of the next 

 flow. Even where a bed of trap is very compact and close-grained, 

 its upper and imder portions are usually cellular and slaggy, the 

 cavities being in most cases filled up with steatite or calc-spar. 

 Hence a section of a series of melaphyre beds presents a succession 



(1862) p. 437) he expresses his doubt 'whether the trap seen near the breccia is 

 interstratified or intrusive. In the course of an excursion into Ayrshire in 1862 

 I descended the course of the Ayr fi-ora Sorn to Stair and recognized the breccia 

 as a true volcanic tuff, or gravelly ash, closely resembling the red trap-tuff of Dunbar. 

 The trap at the same time was foimd to present all the characters of true lava flows, and 

 I saw it coming again to the surface on the west side of the basin. Though it was clear 

 from these sections, that the Permian sandstones of Ayrshire contained at their base a 

 contemporaneous volcanic series, I was unwilling to publish any paper on the subject 

 imtil the whole of the ground had been examined. From the results of this excursion, 

 however, the Permian basin was represented to be bounded by igneous rocks, on the 

 east and west sides, on the Geological Map of the British Isles, published in 1864 in 

 their Educational Series by Messrs. W. & A. K. Johnston, and in the Explanatory 

 Handbook to that map (p. 67) I have referred to the true volcanic nature of the 

 breccia and its associated trap. Since that time, the Geological Survey having been 

 extended into Ayrshire, I have had an opportunity of mapping the Permian basin in 

 detail. The full results will properly appear in the Official Memoirs, and I shall 

 here, with the sanction of the Director General of the Survey, offer a brief summary 

 of them. 



