﻿US 



ME. G. W. TYRRELL ON THE 



[vol. lxxii,- 



The diagram (fig. 5, p. 116) shows the relation of the mineral 

 constituents to depth within the sill. The depths are plotted as the 

 abscissae and the proportions of the various mineral constituents 

 as the ordinates, utilizing the averages Nos. 1, 2, 5, 7, 8, 3, in 

 Table VII, in the order given. The break between the theralite 

 layer and the teschenite, where different types occur on opposite 

 sides of a sharp boundary (between 2 and 5), and the similar 

 break between the peridotite and the lower teschenite (between 8 

 and 3), are clearly shown by the marked changes in the directions 

 of the lines at the points indicated. The sill can thus be divided 

 into three sharply-bounded portions, in each of which a more or less 

 continuous variation may be traced. By far the largest part is 

 that which constitutes the central body consisting of theralite 

 at the top, passing downwards into picrite, and ultimately into 

 peridotite. In this portion plagioclase, analcite, and nepheline 

 show a more or less continuous decrease in a downward direction 

 within the stratum ; while olivine rapidly increases in amount, 

 forming about 65 per cent, of the peridotite, and augite and 

 barkevikite find maxima in the picrite and theralite respectively. 



(2) Chemical Variations. 



The chemical variations within the Lugar sill naturally reflect 

 and follow the lines of the mineralogical variations. Again, as in 

 the preceding section, the variation in 'basicity,' or rather 'mafi- 

 city ' (if one may coin such a term), is the significant variation. 

 In this case, however, it cannot be shown by using the silica 

 percentages as is customary in chemical diagrams, since the silica 

 percentage varies but slightly throughout the series. The percent- 

 ages of the ferromagnesian oxides show a much more sympathetic 



Table VIII. 





1. 



2. 



3. 



4. 



5. 



6. 



SiO, 



45-09 



42-61 



46-40 



42-77 



40-14 



43-16 



TiO-> 



2-95 



2'63 



2-12 



2-73 



1-93 



2-66 



M-ih 



15-49 



14-70 



17-14 



7-48 



325 



10-95 



Fe 2 0 3 



372 



3-37 



312 



4-51 



301 



3-77 



FeO 



7-78 



8-59 



6-49 



10-79 



11-83 



8-97 



MgO 



668 



11-20 



1-98 



16-49 



29-29 



13-84 



CaO 



8-63 



8-54 



7-25 



8-62 



4-40 



7-62 





5-16 



5-91 



' 909 



4-41 



2-43 



4-61 



K 2 0 



V45 



1-14 



0-52 



0-51 



0-43 



1-07 



H-,0 



2-17 



0-15 



4-48 



1-04 



3-10 



2-69 



P."0 5 



0-72 



1-05 



1-36 



076 



026 



0-65 



Rest 





0-07 



0-28 



0-21 



0-15 





Totals 



99-84 



99-96 



100-23 



100-32 



100-22 



99-99 









i 





1. Average of teschenites. 2. Average of theralites. 3. Average of lugarites. 

 4. Average of picrites. 5. Average of peridotites. 6. Average rock "of the 

 Lugar sill. 



