TIUNSACTIONS OF SECTION C. 765 



grained porphyritic rocks, and radiating dikes of aphanitic and glassy lavas, encased 

 in an accumulation of tuffs and breccias, with flows of massive lava, constitute an 

 extinct or completed volcano. The central core consists of magmas that closed 

 the conduit through which many of the eruptions had reached the surface. In 

 solidifying they became coarse-grained. The question naturally suggests itself, 

 Are these rocks any less volcanic than those that reached the surface ? What 

 part of a volcano is n on- volcanic ? 



3. Berthelot^s Principle applied to ALagmatic Concentration. 

 Bij Alfred Harkeg, M.A., F.G.S. 



The paper deals with that type of concentration in which an igneous rock- 

 magma, supposed originally homogeneous, has been difierentiated by accumulation 

 of the more basic ingredients in the cooler marginal part of the liquid. The author 

 tries to find a physical cause for this action by comparing such a magma with a 

 saturated saline solution, and applying Berthelot's ' principle of maximum work ' or 

 the cognate one of ' most rapid degradation.' The migration of the least soluble 

 ingredients to the part of the liquid most easily saturated would determine 

 crystallisation, the process which in the case supposed would give the most rapid 

 evolution of heat. 



4. On the Origin of Intermediate Varieties of Igneous BocJcs hy Intrusion 

 and Admixture, as observed at Barnavave, Carlingford. By Professor 

 W. J. SOLLAS, B.Sc, F.B.S. 



The two principal kinds of rocks composing the mountain of Barnavave are a 

 dark-coloured, almost black, gabbro and a light-coloured, almost white, grano- 

 phyre. This extreme contrast in colour renders the study of their relations to each 

 other in the field a comparatively easy task. The gabbro which overlies the grano- 

 phyre was the first-formed rock, and had ah-eady cooled and solidified before the 

 granophyre was injected into it. The injection of granophyre has been of the 

 most searching character, and the rock can be traced from the parent mass through 

 dykes of all gradations in size down to the minutest films and specks which till 

 cracks and cavities in and amongst the constituent minerals of the gabbro. The 

 gabbro has thus become converted locally into the quartz gabbro of authors, and 

 it is suggested that in other cases, as that of Carrock Fell, this rock has had a 

 similar origin. The granophyre, on the other hand, contains fragments of the 

 gabbro, ranging from great blocks down to mere crystal dust of its constituent 

 minerals, labradorite and augite. It thus passes into hornblendic granophyre, the 

 syenite ' of the Survey. There is no evidence here, as has been erroneously sup- 

 posed, of the diff'erentiation of an originally homogeneous magma, and the minute 

 granophyric dykes are neither contemporaneous nor segregation products. On 

 the contrary, rocks of intermediate character have been produced from already dif- 

 ferentiated and opposed types solely by admixture. 



5. On the Transformation of an Amphiholite into Qtiartz-mica-diorite. 

 By Professor W. J. Sollas, D.Sc, F.B.S. 



On the steep northern side of the upper lake of Glendalough, Co. Wicklow, a 

 coarsely crystalline rock, weathering spheroidally, protrudes in a bold mass through 

 the surrounding Ordovician mica-schists, which it welds at the junction into des- 

 mosite. It consists almost entirely of hornblende, possesses a specific gravity of 

 3-03, and analysed in bulk gives the following results : — 



Silica 48-94 



Alumina 10'54 



Ferric oxide 7 38 



Manganese oxide -1,5 



Lime 10-29 



Magnesia 20-6fi 



Water 2 54 



100-50 



