﻿part 2] 



LTTR.G-ECOMBE MILL LAMPBOPHYRE. 



83 



to the peculiar chemical composition of the inclusions as indicated 

 by their minerals. They were poor in silica, but rich in alumina 

 and iron, and possibly magnesia. In view of this last constituent, 

 the fact that cordierite had not been observed was noteworthy. 

 Possibly some of the patches now consisting of micaceous aggregates 

 were pinite after that mineral. 



Supposing the inclusions to have been shale in the first place — 

 which seemed most likely from the field-evidence — considerable 

 interchange between magma and inclusion must have taken place. 

 Inclusions of Skiddaw Slate in the Threlkeld microgranite offered 

 a marked contrast to this, having been altered, by the addition of 

 a little boron, merely to tourmaline and secondary quartz, a 

 mineralization involving practically no change in bulk-composition. 

 It wo\ild seem that an acid magma, already rich in silica and 

 alumina, had little solvent or metasomatic effect upon argillaceous 

 inclusions, while a more basic magma, deficient in silica but 

 saturated with alumina, had both a solvent and a metasomatic action 

 upon such fragments, the silica being leached out, and its place 

 taken by iron, and to a smaller extent by magnesia, to form such 

 minerals as magnetite and staurolite, while the alumina remained 

 behind in the crystallized condition as corundum. 



Dr. J. W. Evans thought that it would not be right to ignore 

 the possibility that the xenoliths might represent iron-ore from a 

 mineral vein traversed by the intrusion, and thus the corundum 

 might be the result of the removal of silica from the magma in 

 combination with oxide of iron. A still more speculative hypothesis 

 was that they might originate from inclusions of sedimentary 

 rocks, containing free hydrates of iron and alumina, and allied in 

 their nature and mode of formation to laterites. 



The Author agreed with the President that the included 

 fragments were probably derived from some underlying horizon, 

 but considered it unnecessary to assume that this was very different 

 in lithology from, that of the shales now exposed in contact with 

 the intrusion. 



In reply to Prof. Cullis, he considered that the micaceous 

 aggregates of some of the inclusions might represent altered 

 cordierite, but was not prepared to go further. He thought also 

 that it was extremely improbable that analysis of the exposed 

 country rock would give any useful information as to the nature 

 of the material caught up by the magma. Although this material 

 was probably argillaceous, its analysis might differ considerably 

 from that of the exposed shales. 



Replying to Dr. Evans, he said that the extinctions of the 

 plagioclase "cores indicated a composition approximating to that 

 of labradorite. He agreed that the percentage of potash was 

 unexpectedly low — much too low to justify the name of minette. 

 An attempt to incorporate fragments of a laterite would account 

 for some of the inclusions, but in an argillaceous country, probably 

 quite capable of supplying the necessary material, such a mode of 

 origin was only remotely possible. 



