120 MESSES. J. E. DAKTNS AND J. J. H. TEALL ON THE 



side of the Brocken^ mass of granite in the Harz there is a narrow 

 zone in which hornblende-biotite-granite, augite-hornblende- 

 granite, augite-diorite, diorite, and quartziferous biotite-angite- 

 gabbro may be observed. These several varieties shade into 

 each other so that it is impossible to make sharp distinctions. 

 The well-known gabbro of Harzburg occurs to the north of the 

 granite-mass, and associated with it is a quartziferous biotite- 

 augite-gabbro exactty similar to that found in the above-mentioned 

 basic zone. Taking the whole area occupied by plutonic rocks into 

 consideration there is, therefore, every type of rock intermediate 

 between granite on the one side and diorite or gabbro on the 

 other. 



DiSCFSSIOir. 



Dr. Hatch said it was evident that this was a most important 

 paper. Until it was published it would be impossible to discuss the 

 conclusions arrived at ; suffice it to say that they are in accord with 

 the researches of AVadsworth, Brogger, and Yogt. That iron-ore 

 deposits can be formed by processes of segregation from igneous 

 masses during consolidation was a conclusion that would interest 

 both mineralogists and geologists. 



Prof. Bonnet expressed his regret at the absence of the Authors, 

 and his sense of the value of the paper. He thought, however, 

 that the case which they described was somewhat exceptional, and 

 the inferences from it must not be pressed too far, for so far as his 

 experience went transitions from one kind of igneous rock to another 

 were rare. 



Mr. Baerow, in the absence of the Authors, pointed out that this 

 complex formed a part of the great series of granitic eruptions 

 (granitite and diorite), of which the best known were the Dee-side 

 Granite and the Moor of Rannoch Diorite. They were older than 

 the Old Eed Conglomerate, for 'boulders of them occurred in vast 

 numbers in that formation ; while they were more recent than the 

 metamorphism, and part of the normal faulting, of the Central 

 Highlands. The complexes varied very much, in some cases con- 

 sisting mainly of acid rock with a marginal fringe of more basic 

 material (such as the one here described) ; in others the main mass 

 was essentially diorite, more or less completely fringed with 

 granitite. One of the latter type occurred at the head of the South 

 Esk in Forfar. A minute examination had shown that in some 

 cases the acid granitite had been intruded into the diorite in such 

 a way as to leave a very small portion of the latter rock intervening 

 between the former and the bounding walls of the complex, here 

 consisting of gneisses. In such a case the thin patch of diorite had 

 not only been consolidated before the later intrusion, but its tempe- 

 rature was too low for the granitite to re-fuse it, and there was a 

 clear cent act- junction. But on the side of the newer intrusion, 



1 Dr. K. A. Lessen, Zeitschr. d. cleutsch. Geol. Gesellsch. Eand xxxii. (1880) 

 D. 20G. 



