Vol. 50.] OF CAKROCK FELL. 321 



the specimens, on the law that the gahbro becomes more basic 

 from centre to margin. The mineralogical constitution of the 

 rocks varies accordingly. The more acid rocks, in the central 

 part of the area, have plenty of micropegmatite and scarcely any 

 iron ore, while in the more basic rocks of the margin quartz is 

 wanting and iron ores are very abundant, amounting in the extreme 

 case to nearly one-quarter of the whole rock. 



A very little examination of the rocks in the field is enough to 

 convince the observer that the relatively acid and the extremely 

 basic varieties of gabbro represent modifications of one original 

 magma ; that these extremes graduate imperceptibly into one 

 another through intermediate varieties ; and that all the varieties 

 are arranged with striking regularity in successive zones corre- 

 sponding in general form to the boundary of the whole mass. In 

 view, however, of the great difference between the extreme types 

 (the silica-percentage diminishing by 27 in 400 yards) and of the 

 important deductions to be drawn from the phenomena, it is desirable 

 to present the evidence of the continuity of the whole mass in some 

 precise form. To obtain a large number of chemical analyses was 

 impracticable, and I have accordingly availed myself of the density 

 of the rocks as a rough test of their relative basicity, or, more 

 particularly, of their relative richness in the denser minerals. This 

 of course agrees in a general sense with the silica-percentages, the 

 less acid rocks being the heavier ; but, judging from such data as 

 we have, the agreement does not always hold very precisely. The 

 figures for specific gravity are found to show a much more regular 

 distribution than those for silica-percentage. The reason for this 

 is easily seen : the most important respect in which the several 

 specimens differ is in their content of iron oxides, and the best test 



below by the letters S and L, respectively. I give here those which relate to 

 tbe gabbro, and with them the figures from Mr. Barrow's two analyses and 

 from that by Mr. J. Hughes mentioned in Ward's paper. 



(i) White Crags : 



Silica 5946 (S) ; sp. gr. 2-804. 

 (ii) White Crags (the locality may be some distance from the preceding): 

 Silica 59 656 (Hughes), 

 (iii) 350 yds. S. of White Crags, 120 yds. W.N.W. of sheepfold: 



Silica 57-7 (S) ; sp. gr. 2877. 

 (iv) By road-side, about 150 yds. N.N.W. of Chapel Stone : 



Silica 53'50 (Barrow) ; sp. gr. 2 - 800. 

 (v) Same locality : 



Silica 50 - 5 (L). 

 (vi) 600 yds. S.W. by S. of White Crags, 200 yds. B.S.E. of sheepfold : 



Silica 50-22 (S) ; sp. gr. 2939. 

 (vii) 120 yd9. N. of summit of White Crags : 



Sinca 47-11 (S) ; sp. gr. 2"848. 

 (viii) Top of cliff above Mosedale, southern edge of gabbro : 

 Silica 44-14 (S) ; sp. gr. 3-103. 

 (ix) Gill $ mile N.W. of Swineside, southern edge of gabbro : 



Silica 43-4 (L) ; sp. gr. 2-952. 

 (x) Lower part of Furthergill, northern edge of gabbro : 



Silica 33-4 (L) ; sp. gr. 3200. 

 (xi) Upper part of Furthergill, northern edge of gabbro : 

 Silica 32-50 (Barrow) ; sp. gr. 3'265. 



