the Great Whin Sill, Northumberland. 251 



in composition of that great mass of intrusive rock ; in addition they 

 show that the peculiarities of the inclusions are not due to local 

 variation in the sill as a whole. 



Comparing the analyses I and II, it will be seen that silica and the 

 alkalies have increased in the inclusions at the expense of iron, lime, 

 and magnesia, and of the alkalies, potash has increased to a greater 

 extent than soda. The relations between the whin and the inclusions 

 are thus precisely those which have been proved to exist in rocks of 

 andesitic composition between the rock as a whole and its glassy 

 base or groundinass.^ 



This relationship points to a genetic connexion between the rocks. 

 Assuming for the moment that such exists, then the decrease in iron 

 is much smaller than has been usually observed. Such differences 

 may, however, be expected, since the Whin Sill is more basic than 

 the rocks which have been studied in this light, and there is possibly 

 some difference in the order of crystallization in the two cases. 



From the simultaneous decrease in titanium and iron it might be 

 inferred that the former is present, to some extent at least, as 

 ilmenite, as proved by Teall in the rock of Caldron Snout ; but as 

 the decrease is not by any means proportionate, and as augite is rare 

 in the inclusions, it would appear that some of the titanium in the 

 whin was included in the augite. 



The distribution of phosphorus, in cases such as those we are 

 dealing with, is a question to which but little attention has been paid, 

 though it is one of some interest owing to the position i)f apatite in 

 the order of crystallization of minerals from magmas. The analyses 

 show a very considerable enricliment in phosphorus in the inclusions. 

 Assuming the phosphorus in the rocks to be present as chlorapatite, 

 then the whin contains 68 per cent and the inclusions 1'78 per 

 cent of that mineral. The inclusions thus contain 2 6 times as 

 much apatite as the whin, thougli one might expect the order to be 

 reversed corresponding to the greater acidity of the inclusions. 



There is a general resemblance in composition between the felspars 

 isolated from the Whin Sill by Teall (see column III) and the 

 inclusions, the chief difference being in the iron. If we imagine the 

 felspars mixed with 7 per cent of iron oxide, then the resulting 

 material would be very similar in composition to the inclusions. 

 It may be noted, too, that the ratio of the oxides (Nao, Ko) : CaO 

 in the inclusions (1 : 1*3) is not far different trom that in the 

 felspars (1 : 1). 



Before leaving the discussion of the analvses it may be pointed 

 out that while the Whin Sill resembles in composition the dykes of 

 Hett, Collywell, Crookdene, and Morpeth, the inclusions are very 

 similar in composition to the Cleveland and Acklington dykes, as 

 may be seen by comparison of columns II, IV, and V in the table 

 above. There are indications here of an association analogous to 

 that observed in Skye, where dykes of augite-andesite, similar in 

 composition to the Acklington and Cleveland dykes, frequently 



^ See J. J. H. Teall, British PetrograpliTj , 1888, pp. 42, 43, also, among 

 others, J. W. Judd, Q.J.G.S., 1890, p. 341 ; 1893, p. .536; and A. Harker, 

 The Tertiary Igneous Rocks of Skye, 1904, pp. 331, 339. 



