132 MR. ALFRED HARKER OH THE CARROCK FELL [May 1895, 



cession of the Ordovician lavas and the associated intrusive rocks in 

 Caernarvonshire. 1 It may have an application also to a system of 

 sills or laccolites of cognate origin, such as those belonging to the 

 earlier (probably Ordovician) system of intrusions in the Lake 

 District. 2 In such cases we are dealing with differences of level of 

 many thousands of feet, and it is doubtful whether differentiation 

 of this kind could produce appreciable effects within the limits of a 

 single body of eruptive rock such as can be exposed for our exami- 

 nation. Even if the parallel between a rock-magma and a saline 

 solution be granted, physical researches on solutions do not give us 

 any quantitative results available for our purpose. In simple solu- 

 tions Uouy and Chaperon 3 have calculated that the concentration 

 by gravity would become sensible only in a vertical column at least 

 100 metres in height. On the whole, we cannot at present look to 

 this process as likely to have been an important cause of different- 

 iation in a laccolite of moderate dimensions, such as the Carrock 

 Fell granophyre. Moreover, there are facts for which it offers no 

 explanation. 



The idea of minerals separated out in the early stages of crystal- 

 lization sinking to the bottom of an otherwise fluid magma is also 

 one that has been entertained by other writers, and I have else- 

 where applied it to some of the later (post -Silurian) intrusions on 

 the border of the Lake District. 4 It does not seem, however, to 

 afford an explanation of the facts in the present case. In the 

 intrusion of Carrock Pell itself the basic modification of the grano- 

 phyre is certainly found along the southern, i. e. the lower, 

 boundary ; but if we consider the other granophyre-intrusions in 

 the neighbourhood, already enumerated, we see that no such rule 

 holds good. We cannot then ascribe the richness in augite, iron 



1 ' The Bala Volcanic Series of Caernarvonshire,' Cambridge, 1889, § viii. 



2 The following are arranged in descending order : — 



(i) Sill of quartz-porphyry between Ashgill Shales and Coniston Limestone, 

 Far Old Park, Dalton-in-Furness ; sp. gr. 2\574. 



(ii) Sill of microgranite in Coniston Limestone, Taith's Gill Bridge, near 

 Sedbergh ; sp. gr. 2 - 590. 



(iii) Sill of quartz-porphyry in banded ashes near Long Crag, Wrynose 

 Gill ; sp. gr. 2-597. 



(iv) Sill below banded ash-group on west flank of Helvellyn ; sp. gr. 2"726. 



(v) Irregular sill of dolerite near junction of Volcanic Series with Skiddaw 

 Slates, Lyulph's Tower, Ullswater ; sp. gr. 2'803. 



(vi) Irregular sill or laccolite of hornblende-picrite (of Mr. Postlethwaite) 

 in Skiddaw Slates, Great Cockup ; sp. gr. 2 - 923. 



There seems to be no exception in the Lake District to the rule that the 

 lower sills are of denser rocks than the higher. The figures stated are, of 

 course, for the solid rocks, but it is safe to assume that those for the fluid 

 magmas would give the same relative order. 



The vertical range here represented must be very considerable ; Mr. Clifton 

 Ward estimated the thickness of the Volcanic Series alone at 12,000 feet. 



3 ' Sur la Concentration des Dissolutions par la Pesanteur,' Ann. Chim. et 

 Phys. ser. 6, vol. xii. (1887) pp. 384-393. Actual experiments on this point 

 by Bischof and others have not yielded any consistent results. 



4 ' The Lamprophyres of the North of England,' Geol. Mag. 1892, pp. 199- 

 206 ; ' Porphyritic Quartz in Basic Igneous Rocks,' ibid. pp. 485-488. 



