53 



or amorphous (microfelsitic or glassy). The differentiation is due to the fact 

 that after the formation of the porphyritic minerals the conditions of con- 

 solidation were changed by the eruption or intrusion of the mass or 

 by some other cause. If the porphyritic minerals be compared with 

 each other it will be seen that they have crystallised in the order of 

 increasing acidity ; and so also with regard to the minerals of the ground- 

 mass. But if the rock be considered as whole the fact which strikes one is 

 that there has been a recurrence of phase. Minerals of the second and third 

 groups and even quartz, as in the case of the quartz-porphyries, may have 

 been produced at two distinct periods in the process of crystallisation. 

 PROF. ROSENBUSCH concludes by proposing that henceforth the term granular 

 should be applied to rocks in which the process of crystallisation has been 

 continuous, and the term porphyritic to rocks in which there has been re- 

 currence of phase in the sense explained above. 



In the discussion of this important paper the first point that demands 

 attention is the statement with reference to the order of crystallisation That 

 the law of decreasing basicity is true in a broad and general way, 

 in the case of many rocks, will in all probability be admitted by all 

 petrographers ; at the same time it cannot be doubted that exceptions 

 are very numerous. Some of these exceptions may, as Professor ROSENBUSCH 

 points out, be apparent rather than real ; and due to the fact that, in the 

 present unsatisfactory state of our knowledge with regard to the molecular 

 constitution of the complex silicates, we are not in a position to define with 

 precision the meaning of the term basicity. If we compare the unisilicate 

 olivine (ratio of oxygen combined with Mg. and Fe. to that combined 

 with Si. = 1 : 1) with the bisilicates augite, enstatite, &c. (ratio of 

 oxygen combined with Mg., Fe. and Ca. to that combined with 

 Si. = 1 : 2) the law is strikingly illustrated ; for olivine appears always 

 to be the first formed mineral when it occurs in the presence of the bisilicates 

 in ordinary igneous rocks. In the case of the felspars the law is also true 

 as a general rule. Thus Fouaufi (1) has shown that the porphyritic felspars 

 in the Santorin lavas are labradorite (oxygen ratio = 1 : 3 : 6), or in some 

 cases anorthite (oxygen ratio = 1:3:4), whereas the felspars of the 

 groundmass have approximately the composition of albite (oxygen ratio = 

 1:3: 12). 



Optical and chemical researches by subsequent observers have tended to 

 establish the general truth of the law here referred to, at any rate in the 

 large and important group of rocks to which the terms andesite and 

 porphyrite are applied. The earlier porphyritic felspars are usually anorthite, 

 bytownite or labradorite ; the later microlitic felspars are oligoclase or albite. 

 Moreover when these rocks possess any residual glass this substance, as has 

 been already shown, differs in composition from the entire rock exactly as 

 it would on the assumption that the law of ROSENBUSCH is correct. Where 

 monoclinic and triclinic felspars are both present, the latter are usually seen to 

 have been first produced. 



(1) Santorin et ses Eruptions. Paris, 1879. 



