Correspondence — A. J. Juhes-Browne. 141 



and is daily carried into the lake, whose water has thus become 

 turbid and greenish in colour. The rock excavated along the copper 

 ■veins is of a greenish colour, as may be seen by looking at the tips 

 from the adit-levels. 



Tbis change of colour in Llydaw explains the colour of Glaslyn, 

 about the cause of which there has hitherto been some doubt. For 

 it cannot now be doubted that Glaslyn owes its green colour to 

 the detritus of green rock washed into it from the adit-levels of 

 the mines. J. E. Dakyns. 



P.S. — I should say that the mines are situated immediately above 

 Glaslyn. 



Snowdon View, Nant Gwtnant, Beddgelert. 

 January list, 1903. 



THE TERM 'HEMERA.' 



SiK, — Mr. Buckman appears to think that stratigraphy is nothing 

 but geological chronology i.e., that it is chiefly concerned with the 

 days and weeks of geological time, and that the actual sequence of 

 rocks is of less importance. 



He will not admit that his definitions of the term hemera, or his 

 correlation-table of zones and hemerse in Quart. Journ. Geol. Soc, 

 vol. xlix, p. 519, are open to misconstruction, and yet he complains 

 that most of those who have essayed to use his term have misunder- 

 stood the meaning he intended to give it. It now appears that in 

 that table he was giving us a geological calendar, and not an ordinary 

 correlation-table of rock-subdivisions. 



Tlie real fact is that Mr. Buckman gave a name to an abstract 

 idea relating to a thing which had no definite name at the time when 

 he wrote. His paper was a stratigraphical one, and he cannot deny 

 that he was actually dealing with the subdivisions of zones, yet, 

 instead of proposing a name for the small subdivisions which he 

 recognized in the sequence of deposits, he gave a name to the time 

 occupied in the formation of each subdivision; in other words, he 

 saw no necessity to give a name to the thing itself, but only to the 

 geological day or week in which it was formed. 



He asserts that he was giving a name to the duration of a zone, 

 but this assertion is inconsistent with his original definition of 

 a hemera ; he says, " successive hemerse should mark the smallest 

 consecutive divisions which the sequence of different species enables 

 us to separate in the maximum developments of strata." Now, a zone 

 is not the smallest possible subdivision of a series of beds, and 

 Mr. Buckman's own tables show that he knew it was not, for they 

 show that it took the time of two or three hemerae to form one zone. 

 Hence, if a hemera is anything at all it is not the duration of a zone, 

 but of some subdivision of a zone. 



The only point that Mi*. Buckman has made quite clear is, that he 

 will not have his term ' hemera ' used as the name of a rock-division, 

 but he has not clearly indicated with what recognized subdivision of 

 a stage he wishes the term to be connected. If he makes any reply to 

 this letter, let him state clearly whether he accepts the term subzone. 



