68 



occurs as an inclusion in the granite in the same series of 

 exposures as that from which the other specimens were 

 taken. The field relations show that it has been derived 

 from variety No. 2 by further alteration. Although not 

 differing much from No. 2 in chemical composition, under 

 the microscope it is seen to differ considerably from it in 

 structure, the individuals of the several constituents 

 showing a less marked tendency to a rounded outline than 

 in the case of No. 2. In mineralogical composition also it 

 presents certain differences, the pyroxene and scapolite 

 having disappeared and a certain amount of biotite having 

 been developed. 



A comparison of the analyses shows that the granite at 

 first transfuses into the limestone, silica, alumina, oxides of 

 iron and magnesia, with some alkalies and a small amount 

 of titanic acid. As the alteration progresses, all these 

 constituents continue to increase in amount. But in these 

 later stages of the alteration the alumina, oxides of iron, 

 and alkalies are added in relatively greater proportion 

 than the other constituents, while no further addition of 

 magnesia or lime takes place, the proportion of these 

 constituents remaining essentially the same, the carbonic 

 acid escaping and carrying the rest of the lime with it. 



This means, speaking generally, that pyroxene and 

 some scapolite were first developed in the limestone and 

 that later the feldspathic constituents increased in amount, 

 the calcite present being removed in solution. 



A calculation of the analysis shows that Nos. 1 (b) 

 and 2 have the following mineralogical composition: 



— 



No. 1 (b) 



No. 2. 



Feldspathic constituents 



48-57 



46-63 



3-2 



67-35 



Pyroxenic (iron magnesia) constituents 



26-28 

 5-27 





Water 



98-40 

 1-66 



98-90 



I -00 









100-06 



99 90 



