68 Rocks of Noyang. 



measure on the continuously varying conditions thereby 

 produced. 



In considering what may have been their origin and the 

 mode of their formation, it is evident to me that an expla- 

 nation which disregards the influence of those contact 

 sediments which have been absorbed into the mass of the 

 crystalline rocks will be partial and incomplete. 



The microscopical examination of the igneous rocks of 

 Noyang brings out clearly one marked feature, viz., the 

 decreasing amount of the Fe Mg minerals in the succeeding 

 formations. The occurrence of more basic rocks as dykes 

 at the close of the process is a small feature when com- 

 pared to the whole. The analyses which I have given 

 also show this decreasing amount of RO bases, and this is 

 most marked in the latest of the quartz-porphy rites, which 

 is essentially composed of albite and quartz alone. 



Two explanations might be given of this decrease of basic 

 minerals in succeeding rocks. It might be suggested that 

 the successive emissions of rocks of less and less basicity 

 represent the residues of the original magma from which 

 the RO bases had crystallised out as magnetite, mica, or 

 amphibole, and that thus the latest rocks represent the more 

 acid residuum. But this view fails to account for the 

 amount of sediments which have certainly been absorbed, 

 and suggests that the earlier and more basic rocks owe at 

 least some of their basic compounds to the sedimentary 

 materials which have been absorbed. An inspection of the 

 analyses Nos. 3, 4, 5, and 6 shows that absorption of rocks 

 of such a kind would certainly add a comparatively large 

 increase of the RO bases to the igneous masses. To see 

 what might be the result of such a process, I have calculated 

 and subjoin results on the assumption that equal portions of 

 a magma of the constitution of the white quartz-porphy rite 

 (analysis No. 6) and of a sediment (analysis No. 7) were 

 combined. Such a magma would have a mean composition 

 as given below : — 



