Vol. 60.] IGXEOUS ROCKS OF POXIFSFORD HILL. 4S1 



The following table gives the percentage of silica and specific 

 gravity of varieties of the intrusive dolerite, taken from different 

 parts of the mass : — 



JYo. of Percentage Specific 



specimen. of silica. grav ' • 



28 48-30 ' 2-88 



60 4564 284 



211 4762 2-84 



35 4581 284 



17 5013 2-83 



569 285 



54 2-86 



514 2-86 



Average 47'50 2 - 85 



The following complete analysis of a compact, relatively- fresh- 

 looking sample of the granulitic dolerite (28) was kindly made for 

 me by Dr. C. F. Baker, to whom I am also indebted for the 

 determination of the silica-percentages in the foregoing table : — 



I. II. III. 



SiO., 48-30 49-860 48-8 



TiO~ not est. 1330 sorest.' 



Al.,0.3 19-00 12-751) 18-1 



Fe;d 3 6-72 3-360 3-5 



EeO 3-97 11*380 72 



CaO 8-93 8-710 8*4 



MgO 3-53 4-395 4"9 



Na^O 5-01 5250 3-7 



K,0 2-3S n-570 19 



P 5 — 0-580 — 



H 2 2-05 (ignit.) 2-560 30(ignit.) 



Totals 99-89 100745 1001 



Specific gravity 2-88 2907 279 



I. Compact granulitic dolerite, Pontesford Hill. 



II. Dolerite, Rowley. (See J. J. H. Teall ■ Brit. Petrogr.' 1888, p. 213.) 

 III. Dolerite, Hailstone Hill, Rowley. (Do.) 



By the side of this analysis I have placed two analyses of the 

 dolerite, intrusive in the Coal-Measures of Rowley (Staffordshire), 

 with which the Pontesford rock seems to show some points of 

 resemblance. It will be noted, however, that the iron is mainly in 

 the ferric state in the case of the Pontesford dolerite, and the 

 minerals in the latter rock are not nearly so fresh, in general, as in 

 the Rowley rock. This is particularly the case with the olivine 

 and the felspars, while the augite has not the deep purplish-brown 

 colour that characterizes that mineral in most of the Rowley rocks. 

 In this connection it should be borne in mind that the Rowlev 

 specimens, especially those collected by Allport. were obtained from 



1 Probably due to an absence of titanic acid in the Pontesford rock. Un- 

 fortunately, in the above analysis, titanic acid was not looked for. 



