187 



of andesitic lava and tuff (now porphyrite and porphyrite-tuft) were 

 erupted. The activity was prolonged into Carboniferous times, but a change 

 took place in the character of the volcanic material. Basic rocks were then 

 formed on a large scale. These rocks have been described by Mr. ALLPORT^) 

 and Dr. GEIKIE.^ They form amorphous masses, intrusive sheets, dykes 

 and veins, contemporaneous lava-flows, and tuffs. 



Again, the highest members of the Carboniferous formation contain 

 ntrusive sheets of basic material and volcanic necks. These must be, as 

 Dr. GEIKIE pointed out, either late Carboniferous or post-Carboniferous. 

 It is difficult to fix their exact age. Dr. GEIKIE refers them somewhat 

 doubtfully to the Permian period. Lastly, we find in the same district 

 a series of E. and W. dykes which may, with very considerable confidence, 

 be referred to the Tertiary period. 



The basic igneous rocks of the Midland valley are thus seen to belong, 

 in all probability, to three distinct periods ; Lower Carboniferous, Upper 

 Carboniferous or Permian, and Tertiary. Our information as to the petro- 

 graphical characters of the rocks is not as complete as could be desired ; 

 at the same time Mr. ALLPORT and Dr. GEIKIE both agree that there is a 

 close resemblance between the rocks of the different periods. It is possible, 

 however, that differences may be observed between the rocks of different 

 periods when attention is paid to minute peculiarities of structure and 

 composition. We find, for example, in the area under consideration, rocks 

 containing porphyritic augites and a ground-mass composed of minute, 

 well-formed crystals of augite in a felspathic matrix. Such rocks are at 

 present unknown in the Tertiary volcanic district of the West of Scot- 

 land/ 3 ) Again, some of the later intrusive sheets are enstatite-bearing 

 dolerites, allied to the Whin Sill. It is impossible at present to estimate 

 the value of the characters above referred to ; nevertheless the above facts 

 certainly deserve the careful attention of those who possess opportunities 

 for investigation. 



Dr. GEIKIE recognises the existence of six more or less distinct volcanic 

 districts in the eastern portion of the Midland Valley. They are as 

 follows : (1) The district of Edinburgh ; (2) that of East Lothian or 

 Haddingtonshire ; (3) that of West Lothian or Linlithgowshire ; (4) that 

 of Stirlingshire ; (5) that of West Fife ; and lastly (6) that of East Fife. 



In the Edinburgh district lavas and tuffs, interbedded with the 

 Calciferous Sandstone series, form the eminences of Arthur's Seat, Calton 

 Hill and Craiglockhart Hill. The earliest of these belong to the basic, the 

 latest to the intermediate series. Intrusive sheets and irregular masses 

 also occur in the district. Arthur's Seat is a complex mass of igneous 



(1) On the Microscopic Structure and Composition of British Carboniferous Dolerites. 

 Q.J.G.S. Vol. XXX. 1874, p. 553. 



(2) Carboniferous Volcanic Rocks of the Basin of the Firth of Forth. Trans. Roy. Soc. 

 Edin. 1880 p. 486. 



(3) Prof. HULL long ago (G.M. Vol. X. 1873, p. 153,) called attention to a difference 

 between the Limerick (Carboniferous) and Antrim (Tertiary) trap rocks depending on the 

 occurrence of well-formed augite-crystals in the former, 



