Il6 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [May 1894, 



logical scale is suggested, where we may imagine absolute basicity to 

 be represented at one end by magnetite, and absolute acidity at the 

 other end by free silica. The scale actually presented to us by 

 Messrs. Dakyns and Teall, for the illustration of this particular group 

 of rocks, does not reach the extreme possibilities at either end, 

 though the range is very considerable. In the case now before us, it 

 is found that, as the silica increases in a rock, magnesia — after iron- 

 ores the most basic oxide — falls from a high position in the peridotites 

 to almost nothing in the eurite-veins. Out of seven kinds of rocks 

 selected to show the chemical sequence there is only a slight devia- 

 tion from this law in one instance. Lime first rises and then falls, 

 attaining its maximum in the biotite-diorite ; after the fall has set 

 in, it acts in sympathy with magnesia, whilst the iron-oxides follow 

 suit. Alumina rises rather rapidly from a low position in the 

 peridotites to a nearly level position in the diorites and granitites, 

 falling somewhat in the eurite vein-reck. Of the alkalies, soda 

 continues to rise for the most part, but with a somewhat marked 

 fall in the eurite vein-rock. Potash rises throughout the series, 

 except at the point where, as previously stated, there is a slight 

 check in the otherwise uniform descent of magnesia. In the eurite 

 vein-rock, which here represents the acidic termination of the 

 sequence, potash is three or four times in excess of soda. 



That somewhat similar views to those put forward by Dakyns 

 and Teall are entertained by other petrologists may be perceived 

 from a recent paper on the Basic Eruptive Rocks of Gran, in Norway, 

 by Prof. Brogger. That Author confirms statements previously 

 expressed to the effect that the different masses of eruptive rock, 

 which occur within the sunken tract of country between Lake 

 Mjosen and the Langesundsfjord, are genetically connected and have 

 succeeded each other in regular order. In this particular case the 

 oldest rocks are said to be the most basic, and the youngest (with 

 immaterial exceptions) the most acid, whilst between the two 

 extremes Prof. Brogger has found a continuous series. Several 

 bosses of basic plutonic rock lie along a north-and-south fissure-line ; 

 the prevailing form is a medium or coarse-grained olivine-gabbro- 

 diabase ; but pyroxenites, hornblendites, camptonites, labrador- 

 porphyrites, and augite-diorites also occur. Analyses of the typical 

 rocks from three localities on the north-and-south line are given, 

 and the conclusion is reached that the average basicity of the rocks 

 forming different bosses decreases from north to south. 



Of course, it is easy to see that the two cases are not exactly 



