762 REPORT— 1893. 



augen of one or other of the felspars. These consist of crystalline and, usually, 

 very clear and fresh felspar, which has been developed, in the cases specially under 

 consideration, in rocks of detrital origin, from whose original constituents the 

 soluble alkahes had been removed by surface agencies. AVhen such rocks were first 

 acted upon by plutonic agencies they did not, therefore, contain the whole of the 

 constituents out of which new felspar could arise. The question in such a case 

 is. Whence came the alkalies which have combined with the silicates of alumina to 

 form the felspar ? In a few cases it may be surmised that part of the alkaline 

 matter may have been introduced through the agency of percolating waters de- 

 rived from a land-surface, or from the bottom of the sea. In such cases the alka- 

 lies liberated by the weathering are partly returned underground, there to enter 

 upon a new cycle of change. But it is just possible that the chief source of most 

 of the alkalies that are introduced into rocks in process of metamorphism may lie 

 within the inner zones of the earth's crust, whence both potassium and sodium may 

 be expelled in some mobile form capable of diffusing itself throughout certain 

 kinds of rock, and there enter into new combinations in the manner already 

 suggested. 



Augen structure graduates into true pegmatite, in which certain rock-forming 

 silicates have aggregated into zones or bands following the planes of structural 

 weakness in the rock wherein they occur. True pegmatite is thus of subsequent 

 origin to the consolidation of the rock in which it occurs : in this respect, amongst 

 others, it differs from ' giant granites.' Pegmatite, according to this view, is not 

 intrusive in the ordinary sense of the word, but is developed in situ as a conse- 

 quence of local and slight relief of pressure when the parent rock was in a poten- 

 tially molten state resulting from earth-creep under plutonic conditions. 



The continued formation of bands of pegmatite along the dominant planes of 

 weakness in a schistose rock of metamorphic origin must result in the develop- 

 ment of a truly foliated (not schistose) rock, into whose structure crystalline 

 felspar enters as an essential constituent. Such a rock would differ in no respect 

 from a true gneiss. 



If the superincumbent pressure is relieved joer saltum, while any given rock is 

 subjected to a temperature above that of the fusing point of its most refractory 

 constituent, the entire mass enters into a state of fusion, in which form it eats its 

 way upward until a communication is established between the subterranean zones 

 of fusion and the surface, and a volcano is the result. 



It would thus appear that augen structure is one of the first stages alike in the 

 conversion of a metamorphic rock into gneiss, and in that fusion of deep-seated 

 masses which eventually leads to the formation of the non-foliated rocks of erup- 

 tive origin. 



SATURDAY, SEPTEMBER 16. 

 The following Papers and Report were read: — 



1. The Genetic Relations of the Basic Eruptive Bocks of Gran (Krisiiama 

 Region). By Professor W. C. Brogger, of the University of Kristiania. 



This paper dealt with a series of eruptive bosses and laccolites forming a line 

 of hills, of which the chief, in order from north to south, are : — (1) Brandberget, 

 (2) Solvsberget, (3) Viksfjeld, and (4) Dignaes. The main rock type in these 

 bosses was called by the author Olivine-gabbro-diabase. It is basic (43 per cent. 

 SiO.j) in (1), rather less basic (47 per cent.) in (2), and somewhat acid (49 per cent.) 

 in (4). From the intimate connection of the minerals in the different types, and 

 the occurrence of all intermediate varieties, it was proved that these rocks had 

 segregated in succession from a magma whose average composition was not unlike 

 that of the rock of Solvsberget. The gradation in chemical composition produced 



