﻿part 2] THE PICRITE-TESCHEjNTTE SILIi OE LtTGAE. 131 



material. This would indicate that the magma had differentiated 

 in a separate reservoir which was tapped near the top. That was 

 contrary to the visual course of events giving rise to deep-seated 

 intrusions : as a result of folding or faulting of the earth's crust, 

 a fluid magma may come to be at a higher level than the adjoining 

 solid rocks, and the reservoir thus formed is then gradually emptied 

 from a point near its base, where there was the maximum hydro- 

 static pressure ; consecjuently the succession of the intrusions was 

 from basic to acid, as at the Lizard. 



Mr. T. Crook joined previous speakers in congratulating the 

 Author on his clear description of this extremely interesting sill. 

 He asked whether the conditions described held true for only a 

 small portion of the sill, or whether they obtained over a consider- 

 able area. He raised the question, because of its important bearing 

 on the mode of intrusion. It seemed to him that this sill, so 

 far as the particular portion described was concerned, was best 

 explained by successive intrusions after differentiation had taken 

 place. First, the teschenite was injected ; then, before the middle 

 layer of teschenite had completely crystallized, the sill was sub- 

 stantially widened and the peridotite was injected. Finally, and 

 in the same manner, a further slight widening permitted the 

 injection of the ' lugarite ' along the median portion of the sill. 

 He considered it impossible to give a satisfactory explanation of 

 the petrology of -the sill, except by assuming that the widening 

 of the sill took place in three distinct stages corresponding to the 

 intrusion of the three different rock-types described. The assump- 

 tion that liquation had taken place after intrusion raised serious 

 difficulties, and seemed to be an unnecessary complication. 



The Author, in reply, said that he was much obliged for the 

 kind reception accorded to his paper. He thought that the 

 conditions under which magmatic experiments were conducted 

 scarcely approximated to those obtaining in natural magmas, 

 especially in relation to the content of water and other fluxes. 

 He hoped that liquation might yet be experimentally demon- 

 strated in silicate-magmas. With regard to his interpretation of 

 the differentiation of the Lugar sill, he admitted that successive 

 intrusion offered a plausible alternative to liquation, but thought 

 that observations in the field favoured the latter theory. An 

 almost constant feature of composite sills and dykes was the 

 occurrence of xenoliths and xenocrysts along the interior contacts. 

 No such phenomena were to be observed in the Lugar sill, and 

 there were no veins or dykes springing from the interior contacts. 

 Whether liquation was the true explanation of certain features 

 presented by this sill, or not, he thought that the evidence for 

 subsidence of crystals under the influence of gravity remained 

 overwhelming. 



