246 Proceedings of the Royal Physical Society. 



beneath which a mass of ice, instead of being melted, has 

 been kept imprisoned by the overflowing molten mass ; and 

 a case has been observed in the coalfield of Borrowstounness^ 

 on the Forth, 18 miles west of Edinburgh, in which a coal 

 seam, 9^ feet in thickness, was found in a perfectly unaltered 

 condition between two sheets of contemporaneous carboni- 

 ferous basalt, the overlying one having a thickness of 138 

 feet. The heat required to produce the contact meta- 

 morphism around the Harz diabases must have been much 

 greater than that afforded by a contemporaneous lava, which 

 would cool comparatively fast, and give off the greater part 

 of its heat to the surrounding air or water. An intrusive 

 rock would, on the other hand, remain hot for a very much 

 longer period, and could only part with its heat through the 

 surrounding rock, an equal quantity being given off on all 

 sides. Some of the spots of diabase may be sections of necks 

 or pipes up which molten matter continued to rise till it 

 reached a height at which the superincumbent weight of 

 rock was small enough to allow the diabase to spread out 

 laterally in the form of a so-called " laccolite." The altera- 

 tion produced around such a canal would necessarily be 

 much greater than that in the neighbourhood of a simple 

 dyke or stock, as the continuous welling up of molten matter 

 would keep the temperature of the sides of the pipe at a 

 great height till the current began to slacken and the diabase 

 magma to solidify. 



4. Unconformahility of the diabase which is to be seen at 

 many places also proves that the rock is intrusive and not 

 contemporaneous. It is to be seen, for example, in the 

 cutting at the side of the road leading down the west side of 

 the Bodethal immediately below Treseburg. The clayslates 

 in that section are completely cut across by the diabase, 

 which has sometimes caught up portions of the sedimentary 

 strata in a way which clearly establishes its intrusive 

 character. The diabase at Magdesprung on the Selke, 

 N.N.W. of Harzgerode, is also discordant, but not in such a 

 marked degree as at Treseburg. The contact metamorphism 



1 H. M, Cadell, The Volcanic Rocks of Borrowstounness Coalfield (Trans. 

 Geol. Soc, Edinb., vol. iii., 1880). 



