Vol. 51.] MR. H. W. M0NCKT0N ON THE STIRLING DOLERITE. 491 



In all the contact-specimens above described we see evidence of 

 rapid cooling towards the margin of the intrusive rock, whereas in 

 the centre the magma obviously cooled down very slowly. 1 



In general appearance the Stirling dolerite is of a greenish- 

 black colour, with white specks and occasional white and pink 

 patches. There is always, or nearly always, a certain amount of 

 pyrite scattered through it. There is frequently much calcite in 

 veins, and in places the rock when altered becomes serpentinous. 

 The rock has a certain amount of character of its own; thus, 

 although I doubt whether, even assisted by the microscope, one 

 could say whether a specimen came from the Abbey Craig, Stirling 

 Castle, Sauchie, or Kilsyth, yet the dolerite differs more or less from 

 most of the Scottish dolerites with which I am acquainted. Sir 

 Archibald Geikie, however, observes that it is very similar to 

 dolerite at Lindsay's Craig near Kirkliston, Crossal Hill near Dal- 

 meny, and Muchraw near Torphichen. 



A rather fine-grained variety of the Stirling rock from Kilsyth is 

 figured by Mr. Allport, as I have already stated, and an account of 

 the rock will be found in the Memoirs of the Geological Survey of 

 Scotland, Sheet 31, 1879, p. 44 ; but I have seen no drawing that 

 gives any very good idea of the coarse-grained dolerite of which the 

 greater part of the Stirling rock consists. Perhaps the nearest is 

 a figure of ' Dolerite labradorique de Laprabende-Caupenne, Landes,' 

 given by MM. Fouque and Levy. 2 



In conclusion, I ought to thank numerous geologists who have 

 given me assistance and encouragement in the study of igneous 

 rocks in general. As regards the Stirling rock in particular I have 

 several times consulted Mr. Teall when in doubt on various points ; 

 Prof. Bonney, with the greatest kindness, has looked through my 

 microscope-slides and given me many hints, of which I have made 

 full use in the preceding pages ; and Sir James Maitland has 

 accompanied me in much of my field-work, and has lent me several 

 of the microscope-slides described in this paper. 



Discussion. 



Prof. Judd referred to the great interest of the new details given 

 in this paper, and asked for further information concerning the 

 mode of occurrence of the rock containing red felspars and the dark 

 opaque junction-rocks with much magnetite. 



Prof. Bonney said that the sections seemed in some respects 

 analogous with the lower part of those at Arthur's Seat, and it was 

 curious that the intrusive dolerite of Salisbury Craigs bore con- 

 siderable microscopic resemblance to the intrusive masses of Stirling. 

 He agreed with Prof. Judd that the glassy rock mentioned was 

 probably only darkened with magnetite, and referred to other cases 

 of felspars of a red colour both in dolerites and in veins in the 



1 Stecher, ' Contact-phenomena of some Scottish Olivine-Diabases,' Proc. 

 Roy. Soc. Edinb. vol. xv. (1888) p. 160. 



a ' Roches eruptives franchises,' 1879, pi. xxxvi. fig. 1. 



