26 MR. E. H. CUNNIS6HAM-CRAIG ON [Feb. I904, 



chlorite begins to give place to biotite. Under the microscope the 

 albite soon appears cloudy and decomposed, and tin ally, as the junc- 

 tion is approached, vanishes altogether, while biotite and contact- 

 minerals make their appearance, and may become very conspicuous, 

 in the ' hornfelsed ' zone near the contact. Dr. Flett has identified 

 cordierite in the hornfels surrounding the Inversnaid hyperite. 



A specimen (8992, PI. V, figs. 1 & 2). which shows very clearly 

 what happens to the albite-gneiss within an aureole of contact- 

 metamorphism, is taken from the railway-cutting north of Ardlui, 

 at a distance of about lj miles from the great Meall-Garabal 

 complex described by Messrs. Teall & Dakyns. 1 The hand-specimen 

 resembles very closely the normal albite-gneiss with quartz-veinlets, 

 but the colour is darker and the rock generally less fissile. Under 

 the microscope it is seen that albite and chlorite have almost 

 entirely disappeared, the former being replaced chiefly by aggregates 

 of white mica, in which a soda-mica is probably present, while 

 sporadically - developed flakes of biotite replace the sheaves of 

 chlorite. In the siliceous folia the development of biotite is more 

 regular. A few rather decomposed blebs of albite can still be recog- 

 nized in some parts of the slide, and in these cases a little chlorite 

 is generally present also. More significant of the contact-action 

 are groups of andalusite-granules which occur among the feathery 

 masses of white mica." 



IX. Xature of the Albeie-Gxeiss Metamobphibm. 



It is not my intention to go more fully into the contact-action of 

 these igneous masses. Mr. Clough, in the Survey Memoir on the 

 ' Geology of Cowal,' has described the contact-metamorphism on the 

 other side of the same petrographical complex. For my purpose, 

 sufficient has been said to show the effects of a thermal contact on 

 the rocks which have been previously altered to albite-gneisses. 

 This leads naturally to the question as to what is the kind of meta- 

 morphism to which the production of these albite-gneisses is to be 

 attributed. On this question I have to offer a suggestion, which must 

 for the present remain more theoretical than the other conclusions set 

 forward in this paper. I have had the experience of tracing the same 

 rocks of the Beinu-Ledi Group through a progressive metamorphism 

 in other districts, and more especially in the district of Aberfeldy, 

 where they may be traced from not very greatly-altered schistose 

 grits into highly-crystalline muscovite-biotite- schists or gneisses 

 with a considerable number of large and well-developed garnets. 

 The dynamic metamorphism is much the same as in the Loch- 



1 Quart. Journ. Geol. Soe. vol. xlviii (1892) p. 104. 



2 In the andalusite-biotite hornfels near the contact with the Glen-Fine 

 granite, described by Mr. Clough, in the Survey Memoir on the ' Geology of 

 Cowal ' (1897) p. 98, a quantity of albite is still present, but it has probably 

 been recrystallized : idiomorphic outlines are not uncommon, and twinning is 

 frequent. 



