﻿120 



MR. Or. W. TYRRELL ON THE 



[vol. lxxii, 



by Dr. Scott. Thus No. 3 in Table II (p. 104), No. 5, Table IV 

 (p. 110), and No. 5, Table VI (p. 114), have not been used on this 

 score. Furthermore, No. 7, Table II, has been omitted, because 

 it represents merely a small and unimportant schlieren, which 

 would have an effect on the average of the teschenites incommen- 

 surate with its size and importance. 



The sum of the ferromagnesian oxides being utilized as abscissae, 

 the diagram shows that the order of increasing richness in these 

 oxides is lugarite, teschenite, theralite, picrite, and peridotite. The 

 most striking feature of the curves (fig. 6) is the rapid and regular 

 fall in alumina from lugarite to peridotite, and the slight fall in 

 silica. Soda also falls ; but potash, along with phosphorus pent- 

 oxide and titanium dioxide (ignoring small irregularities), remains 

 approximately at a constant level. Lime rises to a maximum in 

 the middle of the series, and drops in the lugarite and peridotite 

 at the extremes. The variation of the curves in this diagram may 

 obviously be correlated with the mineralogical variations which 

 have already been described. 



(3) Average Magma of the Lugar Sill. 



In the question of the differentiation of the Lugar sill it is of 

 importance to know the chemical composition of the magma before 

 it split up into its present heterogeneous parts. Assuming that 

 the variation has resulted from the differentiation of an originally 

 homogeneous magma, the composition may be calculated by 

 weighting the analyses of the various components according to 

 their bulk, adding, and then dividing by the number of units 

 taken. The average composition of the principal types is given in 

 Table VIII. The proportions have been taken as follows : — 



Peridotite (Table VIII, 5) 



Picrite (Table VIII, 4) 



Theralite (Table VIII, 2) 



Teschenite (Table VIII, 1) 



Contact-teschenites (Table VIII, 1) 



Since the analyses of teschenite and of the contact-rocks are for 

 all practical purposes identical, seven parts of Anal. 1, Table VIII, 

 have been taken for the last two items. Lugarite is omitted, as it 

 forms an insignificant proportion of the mass of the sill. The 

 result of the calculation is given in Table VEII, 6. 



These figures possess significance only if the hypothesis of an 

 intrusion of homogeneous magma, followed by differentiation in 

 place, be accepted. On the hypothesis of successive intrusion it is 

 impossible to assume that the relative volumes of the facies have 

 any necessary connexion with the relative volumes developed 

 within the differentiation chamber. If the former hypothesis be 

 assumed correct the figures show that the original magma must 

 have had a composition intermediate between that of theralite and 

 that of picrite. It had a certain correspondence with that of 



