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THE PICHITE-TESCHENITE SILL OF LIT GAR. [vol. lxxii, 



and careful account of this remarkable composite sill would make 

 a valuable addition to our knowledge. The theoretical discussion 

 based on the facts opened up several interesting questions. The 

 Author had proved that there was a discontinuous variation, with 

 a continuous variation superposed upon it. It had been clearly 

 demonstrated that the latter effect was due to the settling-down 

 of the earlier-formed ciystals. With regard to the discontinuity, 

 however, the experiments of the Washington chemists made it 

 difficult to accept any explanation postulating immiscible partial 

 magmas. The speaker would like to see the relations of the 

 various rocks re-examined upon the alternative hypothesis of 

 successive intrusions : the suggestion being that the picrite had 

 been intruded in the midst of an earlier intrusion of teschenite, 

 just as the lugarite was admittedly intruded later in the midst of 

 the picrite. 



Prof. W. J. Sollas complimented the Author on a remarkably 

 thorough piece of work, which gained in presentation by the con- 

 scientious manner in which fact and hypothesis had been kept 

 distinct. He was inclined to think, however, that the sill had 

 been formed by two separate infillings : that the upper and the 

 lower teschenite were parts of the same intrusion, which followed 

 a widening and reopening of the fissure. 



Dr. J. W. Evans welcomed the paper as a valuable contribution 

 to the study of the difficult problem of the differentiation of 

 igneous magmas. He saw no reason for rejecting the supposition 

 that, if it contained sufficient water, a rock-magma might separate 

 on cooling to a certain point into two non-miscible portions, the 

 lighter and uppermost of which contained the greater part of 

 the water, silica, alumina, and alkalies. It was true that no 

 experimental evidence of differentiation had been obtained, when 

 mixtures of silicates had been fused together ; but, in the experi- 

 ments, no considerable amount of water under pressure was present. 

 Continuous gravitational differentiation might be expected to take 

 place in a reservoir of sufficient depth, but the amount of such 

 differentiation that would occur with any particular magmatic 

 composition, temperature, pressure, and depth of reservoir, remained 

 for determination. The succession of the igneous rocks at the 

 Lizard appeared to point to a more or less continuous differentiation 

 from an ultrabasic magma to one with the composition of a 

 dolerite, and then a discontinuous differentiation to a granitic t} r pe. 

 The intrusion of the two latter together gave rise to the Kennack 

 gneisses, in which they remained distinct, except where the loss of 

 water before complete consolidation permitted local diffusion. 

 Differentiation as the result of crystallization, in the manner 

 described by Dr. Harker in his 'Natural History of Igneous Rocks,' 

 was now universally accepted. 



In the case of the sill described by the Author the possibility of 

 successive injections could not be ignored. If this had happened, 

 the teschenite must have been first intruded and then, before it 

 had completely consolidated, it was followed by still more basic 



