Vol. 50.] OF CARROCK FELL. 329 



the later stages of consolidation in acid lavas : while the phenomena 

 of differentiation are most strikingly exhibited in the earlier stages 

 of consolidation of basic intrusive rocks. The best experimental 

 results bearing on the question are those of Vogt, 1 obtained from 

 artificial slags having the general composition of igneous rocks. 

 Any differences that exist between the conditions in the artificial 

 and the natural magmas will probably tend to lower the viscosity 

 in the latter, which may be expected to contain a certain amount 

 of water in all cases, and sometimes other fluxes (' agents minerali- 

 sateurs '). Yogt found viscosity to be in direct relation to acidity, 

 the results differing widely for extreme cases. At the same 

 moderate temperature-distance above their respective melting- 

 points, strongly basic slags flow like water, while strongly acid ones 

 are as stiff as tar. The ' melting-point ' of a rock-magma is not 

 a term of precision ; but, so far as our knowledge goes, it seems 

 highly probable that diffusion can proceed freely during the earliest 

 stage of crystallization in a basic rock-magma, being, however, 

 checked more and more as the temperature falls. 



6. Some Deductions from the Phenomena. 



I have dealt rather fully with the gradual variation of the gabbro 

 from centre to margin, because the results, if they are considered to 

 be established, bear upon theoretical questions which have lately 

 attracted much attention. But these results also have very 

 direct consequences for the particular area under discussion, leading 

 to certain definite conclusions as to the geological relations of this 

 gabbro intrusion, and enabling us to discard confidently certain 

 suggestions that have been made under this head. Similar 

 reasonings will be applicable to other masses of igneous rocks 

 showing a like type of differentiation. The several points are 

 sufficiently obvious to be treated summarily. 



Firstly, then, the gabbro is a true igneous rock. The whole 

 body of it was at one time fluid enough to admit of the freest 

 movement among its parts, and that too while all tJie surrounding 

 rocks were cool. This consideration, we think, is enough to dispose 

 of the theory of Mr. Ward, 2 that the gabbro has been produced by 

 extreme metamorphism of the volcanic rocks. Chemical and other 

 facts equally militate against such a hypothesis. 



Next, we see that the gabbro mass does not represent any 

 portion of a duct of a volcano, but is an intrusion of laccolitic type. 

 The magma was injected among cool rocks, and there consolidated. 

 Had there been any prolonged flow of molten matter, the sur- 

 rounding rocks must have become heated, and the mass that finally 



1 Zeitschr. fiir prakt. Geol. vol. i. (1893) p. 275. 



2 Ward quotes from Sedgwick a passage in the same general sense, though 

 offered merely 'as a conjecture' (third Letter to Wordsworth, 1842). 

 J. G\ Marshall maintained the Carrock Fell rocks, with all the chief igneous 

 masses of the Lake District, to be of metamorphic origin (Keport Brit. Assoc. 

 1858, Trans. Sect. p. 84.) 



