36 A. J. Jukes-Broivne — The Term ' Semera.' 



Under crossed nicols the mosaic is seen to consist of areas of 

 uniform extinction, and the included bubbles bear no relation tO' 

 the lines of shear. 



We are here forced to conclude that the parts have once been 

 continuous, that the liquid inclusions are original features, and that 

 they are portions of veins which have been disconnected by earth 

 movements. 



If the quartz, when traversing the granite, is of igneous origin, 

 and the formation locally of a pegmatite is proof on this point, we 

 cannot deny a similar origin to the quartz occurring as veins in 

 physical continuity with that in the granite. 



There is a priori no reason why quartz should not exist as an 

 igneous rock. Given a magma with a limited amount of bases, 

 combinations would go on until the silica had united with all the 

 bases available, and then a residuum would be left which on 

 consolidation would be pure quartz. 



A comparison might be made of these veins with the Pfahl of 

 Bohemia, where quartz occurs as veins through the granite and 

 mica-schists, extending in unbroken sheets over a distance of 

 55 kilometers. Giimbel, however, has shown that the Pfahl is 

 associated with displacement, and he regards the quartz as filling up 

 cracks after faulting. In the Isle of Man, I am inclined to look 

 upon the veins as true igneous dykes, running parallel to the micro- 

 granite intrusions, and only differing from them in their exceptional 

 composition. 



IX. — The Term ' Hemera.' 

 By A. J. Jukes-Browne, B.A., F.G.S. 



IN the December number of this Magazine Mr. Buckman complains- 

 that the use of his term hemera has been widely misunderstood. 

 He says that it was never intended to be used for a subdivision of 

 a zone, but solely as a chronological term to indicate the time during 

 which the beds composing a zone were deposited, just as the term 

 age is now generally accepted in a technical sense to mean the time 

 during which a stage was deposited. 



As one of those who must plead guilty to having misunderstood 

 Mr. Buckman, I should like to explain how it is that he seems to 

 have laid himself open to misapprehension. To put it briefly, 

 Mr. Buckman, in his original paper of 1893, used the term hemera, not 

 merely as a chronological term (though he defined it as such), but 

 also as serving to represent a subdivision of a zone ; for in two of 

 his tables he used it as a stratigraphical division, and consequently 

 it is his own fault if others supposed he meant it to be used in a 

 stratigraphical sense, in spite of his own words about its chronological 

 meaning. 



That this was the immediate effect of his paper is proved by the 



report of the discussion upon it, in which the President spoke of 



" subzones or emata," ' and Mr. Winwood protested against the 



proposed term, saying that "if Mr. Buckman found it necessary to- 



' The word originally suggested was emar, subsequently altered to hemera. 



