part 2] geology or the marble delta. 137 



EXPLANATION OF PLATE IX. 



Fig. 1. Soda-granite. Q, quartz ; O, orthoclase and A, albite, forming 

 perthite ; SA, soda-augite ; Sa, soda-amphibole. Magnification, 

 22 diameters. (See p. 127.) 



2. Forsterite-spinel-marble. C,calcite; F,forsterite, partly serpentinized; 



S, spinel; Sp, sphene. Magnification, 48 diameters. (See p. 131.) 



3. Chondrodite-marble. C, calcite ; M, phlogopite-mica ; F, forsterite ; 



Ch, chondrodite. Magnification, 22 diameters. (See p. 131.) 



[All under natural light.] 



Discussion. 



Dr. J. W. Evaxs remarked on the interest of the facts disclosed 

 by the paper. The disruption of the intrusion without any 

 corresponding evidence of dislocation in the surrounding crystalline 

 limestone was to be explained by the facility with which calcite, 

 by means of its gliding planes, accommodated itself to the forces 

 impressed upon it, so that a saccharoidal limestone might almost 

 be said to be capable of a kind of slow, highly-viscid now. The 

 speaker suggested that, in this case, the ciwstalline limestone might 

 be found, on more detailed examination, to show considerable 

 variation in its magnesia-content, even where it had not suffered 

 any change from the intrusive magma. Mr. Crook had shown 

 that rocks composedof calcite, dolomite, and magnesia respectively, 

 or intermediate types, might be indistinguishable in hand-specimens, 

 except by means of their specific gravity. The evidence afforded 

 by the chemical changes in the crystalline limestones of the 

 volatile constituents, which must have been present in the magma 

 when it was intruded, was of the greatest importance. 



Dr. F. H. Hatch, who was unable to be present at the reading 

 of the paper, wrote to congratulate the Author on a valuable 

 contribution to an interesting subject. The view that the granite- 

 fragments were older than the dolomite which contained them, 

 postulated subsequent heating to explain the chemical interaction 

 of which evidence was furnished by the 'reaction-rims' l ; and this 

 had always been a difficulty. If. the Author had been able to 

 prove that these apparent ' boulders ' were in reality parts of 

 intrusive tongues, he could offer a much simpler explanation of the 

 facts, and one which the writer (Dr. Hatch) would be prepared 

 to accept. 



1 See Q. J. G. S. vol. lxvi (1910) fig. 3, p. 512. 



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