900 MR JOHN S. FLETT ON 



In all three, crystallisation began in the same manner. Perofskite, magnetite, apatite, 

 olivine, augite, were the first products. In the alnoite the augite was accompanied by 

 much biotite, conditioned, no doubt, by the excess of magnesia and of potash (anal. 4), 

 and these two went on crystallising till the still liquid residuum was rich in lime and 

 comparatively poor in alumina and magnesia (anal. 5), crystallising finally as melilite. 

 In the melilite monchiquite the conditions were less favourable for the production of 

 biotite, and for some time augite alone was formed, till a similar condition supervened, and 

 crystallisation was completed by the formation of melilite. But, in the monchiquite, 

 owing to the higher percentage of silica and lower percentage of lime and magnesia the 

 continued crystallisation of augite was giving rise to a residuum comparatively acid, 

 containing the alkalies in fair amount and in nearly equal proportions, and practically 

 devoid of lime, iron, and magnesia {cf. anal. 1). To its peculiar chemical composition 

 wc must ascribe its final solidification as a glass. As in the closely-related rocks 1 and 

 3 an anhydrous crystalline silicate was finally formed, there does not seem to be any 

 reason to believe that in this one, had crystallisation taken place, the water would have 

 been retained, and the hydrous silicate analcite have been the result. 



Conclusion. 



It will be seen that in the Orkneys we have one of the most abundant and interest- 

 ing series of the camptonite-monchiquite-alnoite rocks which is anywhere known to 

 exist. They fall naturally into two groups, the " leucocrate " rocks of Brogger, of 

 which the sole representative is one dyke of bostonite, and the " melanocrate," to which 

 the others belong. Between them there is no connecting link ; and the theory which 

 he expounds of their origin by complementary differentiation from the same magma, is, 

 in view of their constant association, the most natural that has been suggested. 



The " melanocrate " series must be regarded as a unity. The series of gradations by 

 which they merge into one another is so complete that they cannot be separated. They 

 have proceeded from one magma, which, by a progressive differentiation, was becoming 

 more and more basic as time elapsed. Taking the Orkney dykes as a whole, the petro- 

 graphical characters agree with the facts regarding their geological occurrence, in 

 establishing that they have been emitted, no doubt in successive periods, from the same 

 volcanic focus. 



But when we pass to consider what the nature of that parent magma has been, it 

 must be insisted that there are no data, within the Orkneys, which will enable us to 

 decide. In chemical composition they correspond exactly to no plutonic magma, and 

 seem to be in every case products of differentiation. They are typical dyke rocks, and 

 occur only as dykes or thin intrusive sheets. It may be possible, when their distribu- 

 tion in the north-east of Scotland is better known, to trace them to some parent mass ; 

 till then we can argue only from general facts. From what is known of this series at 

 the present day, it is difficult to point to any magma from which we could say with 



