﻿part 2] 



PICRITE-TESCHEjSTTE SILL OF LTJGAE. 



115 



present heterogeneity of the sill suggests a very complex process of 

 differentiation. Moreover, if it were intruded as a homogeneous 

 body of magma the chemical composition of the contact-rocks 

 should be similar to the bulk-composition arrived at by averaging 

 the analyses of the different parts after weighting them according 

 to their volumes. As will be seen later, this is by no means the 

 case ; and, consequently, if the sill is the result of a single act of 

 intrusion, the magma must have been heterogeneous prior to in- 

 trusion. The question of its differentiation is then shifted back 

 to an ante-intrusion stage, and its discussion becomes correspond- 

 ingly difficult. 



These complications are avoided, to some extent, if the mass be 

 regarded as a composite sill resulting mainly from two acts of 

 intrusion — the first introducing the teschenite at both contacts, the 

 second bringing in the ultrabasic rock of the interior. All phases 

 within the sill, however, are intimately welded together, showing 

 that the later intrusion must have quickly followed the earlier. 

 Moreover, the mineral and chemical composition of the rocks show 

 that the successive intrusions have very close genetic relations, and 

 have probably arisen . by the differentiation of a single body of 

 magma. The problem of the mode of differentiation then becomes 

 the same as that arising from the first hypothesis. 



If the sill be thus composite, it shows a surprising lack of xeno- 

 crysts and xenoliths, or of veins and dykes, along the main interior 

 contacts. Other features, too, are difficult of explanation on this 

 hypothesis. The subsidiaiy differentiation observed within the 

 central ultrabasic stratum seems most easily explained by the 

 hypothesis of sinking of early heavy crystals under the influence 

 of gravity, aided perhaps by a concomitant rise of lighter con- 

 stituents. 



The special features of the Lugar sill will now be treated in 

 detail, especially with regard to their bearing on the hypotheses 

 outlined above. 



(T) Mineralogical Variations. 



Estimations of the modes of twelve types of rock occurring in 

 the Lugar sill are scattered through the foregoing petrographical 



discussion upon the paper, and communicated privately ; and also on account 

 of the views on differentiation in general expressed by 1ST. L. Bowen in a recent 

 important paper (' The Later Stages in the Evolution of the Igneous Rocks ' 

 Journ. Geol. Chicago, vol. xxiii, 1915, Suppl. pp. 1-91). Liquation theories of 

 differentiation stand in need of drastic revision after the evidence brought 

 forward in this paper, broad-based upon the exact experimental work carried on 

 for many years at the Geophysical Laboratory of Washington, that liquation has 

 never yet been observed in the melts experimented with. Neither has it been 

 observed in lavas, which are Nature's ' quenching experiments.' Whether we 

 can argue from experiments upon the comparatively minute laboratory scale 

 to age-long magmatic processes under natural conditions, is at least open 

 to doubt; and, until that doubt is resolved, it is permissible to use liquation 

 hypotheses, but to assign to other hypotheses more weight according to their 

 correspondence with known facts and conditions.] 



