W. M. Hatchings — An Interesting Contact-Rock. 129 



Since writing the former paper I have examined specimens of it 

 on a muoh better scale than any I had then seen. Thus there are 

 occurrences of shale at contact with, or enclosed in the Whin Sill 

 mass, which have been left altered mainly, and in one case almost 

 wholly, to this condition. One such rock gives sections which are 

 made up entirely of a ground-mass of this fine speckly material, not 

 far removed from an isotropic condition, with a good amount of 

 white mica and a little dark mica crystallizing out of it in radial 

 bunches and fan-shaped aggregates. Not a vestige of anything of 

 the original shale has remained, except the ever present zircons and 

 the abundant re-crystallized rutiles. This has been a shale rather 

 nnusually rich in alkalies (over 7 per cent.), and very free from 

 quartz. It has been extra amenable to solution-action, and it is not 

 possible to study thin sections of it in its present condition and 

 doubt that very nearly all its ingredients had passed into some sort 

 of solution, and that the indefinite and amorphous product was not 

 very far advanced in a process of re-crystallization when fall of 

 temperature put a stop to further development. 



It is assuredly a circumstance worthy of note, that in the contact- 

 metamorphism of rocks of the same general nature (clays, shales, 

 and slates) by intrusions of granite, and again by intrusions of 

 dolerite, we can find one and the same characteristic substance 

 appearing, and playing one and the same part in its relationship to 

 the formation of certain minerals, this substance being an absolutely 

 new product never seen in the original rocks, nor seen, so far as 

 I am aware, in anything but this particular class of contact-rocks. 



Finally, to return to the special bed of rock at Falcon Glints, we 

 may look again at the analysis. It is not intended here to enter 

 into any detailed consideration of the interesting chemical questions 

 involved in the alteration of shales, etc., by basic rocks, which 

 questions will be more fully gone into at some future time. 

 I may, however, briefly allude to the point as to the increase 

 of soda in such altered rocks. That such increase has frequently 

 taken place is quite beyond question, as I pointed out in my former 

 paper, unless we wish to absolutely reject much very excellent 

 evidence.^ Many of the altered shales of the Whin Sill contact will 

 also bear testimony to the same effect. It is not possible in this 

 case to compare analyses of any given stratum of the rock at 

 successive stages of approach to the actual contact, because the 

 Whin Sill is intruded parallel to the strike, and the thickness of any 



' High authorities, in reviewing all the evidence, have had to come to the con- 

 clusion that such transfer takes place at basic contacts ; as, for example, Zirkel in his 

 detailed consideration of the contact-effects of diabases (Petrographie, vol. ii. 1894), 

 and also Roth (Chemische Geologie, vol. iii. 1893). No geologist has made such 

 a special study of the chemical side of petrology as Roth, and a great amount of the 

 evidence on this particular question is collected and summarized in the volume 

 named. In reviewing this evidence Roth says that, according to it, "there is a 

 decided contrast between the contact- eifects of granite, syenite, etc., and those of 

 diabase : in the former case no introduction of material into the altered slates and 

 no chemical differences due to distance from the eruptive rock ; whereas in contact 

 with diabase we have introduction of silica and soda into the altered slate, and at 

 the same time chemical difference according to distance from the eruptive rock." 



DECADE IT. — VOL. II. NO. HI. 9 



