﻿part 2 J 



PICRITE-TESCHENITE SILL OF LTJ&AK. 



113 



The other variety is an extremely fresh rock, with abundant 

 olivine and some fresh plagioclase. It occurs between the perido- 

 tite and theralite in the great cliff of the Grlenmuir Water. It 

 differs from the rock described above in the much greater abundance 

 of olivine, in less analcite and turbid decomposition-products, and 

 in the presence of plates of fresh felspar enveloping olivine and 

 augite, and showing the usual expansion-fissures. 



Quantitative Mineral Composition of the Picrites 

 and Peridotites. 



Table Y records the results of the Rosiwal measurement of three 

 of the ultrabasic rocks of the Lugar sill. 



Table V. 





I. 



ii. 



III. 



Plagioclase (AbiArii — AbiAn<>) 





57 





Analcite 



13-4 



8-8 





Titanaugite 



56-6 



261 



20-5 



Barkevikite 



127 



8'6 



io-o 



Olivine : 



11-1 



49-1 



65-2 



Biotite 





0-4 



2-0 



Titanifevous iron-ore 



51 



l'O 



2-3 



Apatite 



ri 



0-3 





I. Augite-picrite, upper part of ultrabasic stratum, Glenmuir Water. 

 II. Olivine-picrite, same position and localit.v. 

 III. Hornblende-peridotite, main mass of ultrabasic stratum, Glenmuir Water. 



It will be seen that the picrites are domafic, and the peridotite 

 permafic. The analcite totals contain a little unidentifiable turbid 

 matter. The diminution in the amount of apatite is a curious 

 feature of the ultrabasic rocks. In the Lugar series, and in the 

 rocks of the Ayrshire petrographical province generally, apatite 

 seems to vary in abundance along with the felsic minerals, especi- 

 ally analcite. A similar but less marked decline in the amount of 

 titaniferous iron-ore also occurs. The order in which the analyses 

 are given is that of increasing depth in the ultramafic stratum. 

 The rapid increase in the amount of olivine illustrates the packing 

 of the olivine-crystals in the lower part of the stratum by settling 

 under the influence of gravity. Along with this there is a decrease 

 in the amount of augite ; but barkevikite, a mineral of later con- 

 solidation in these rocks, remains practically constant. 



Chemical Composition of the Picrites and Peridotites. 



Chemical analyses of the augitic type of picrite and of peridotite 

 were made for me by Dr. A. Scott ; and these are supplemented by 

 analyses calculated from the quantitative mineral compositions 

 recorded in Table V, above. 



