430 Correspondence — Mr. Mark Stirrup. 



firmed by the occurrence of the Archasocyathinaa, which are likewise 

 associated with similar Cambrian Trilobites in New York and 

 Canada. In Dr. Woodward's paper the relationship of the Archao- 

 cyothus forms was not recognized, but they were supposed to be 

 Corals. 



Mr. Etheridge further describes some peculiar microscopic tubuli, 

 referring them doubtfully to Girvanella. Similar ftmns have been 

 noticed by Dr. Bornemann in the Cambrian strata of Sardinia. 



This paper is an important contribution to Australian geology, as 

 it definitely proves the existence in that continent of a well-marked 

 horizon of Cambrian rocks closely corresponding to the Lower 

 Cambrian of the Northern Hemisphere. 



COREESPOITDEITCE. 



WIND WAVES AND TIDAL CURRENTS. 



Sir, — Mr. T. Mellard Eeade, in putting before the readers of the 

 Geological Magazine his views on the origin of tbe Lower Trias 

 (Geol. Mag. Feb. April, and June, 1890) drew from Mr. Arthur 

 E. Hunt, F.L.S., a letter on " Tidal Action," in which the latter 

 denies the power of tidal currents to do the work invoked by Mr. 

 Eeade in his theory of the marine origin of the pebbles of the Bunter. 



Mr. Hunt writes (Geol. Mag. April, 1890, p. 191) as follows : 

 "It may be well to point out one line of evidence which seems to 

 have been overlooked by the supporters of the tidal theory, i.e. the 

 zoological." He gives the English Channel as an excellent test 

 case, and remarks, that "if unchecked tidal currents are anywhere 

 resistless, they should be so here. Do these tidal currents disturb 

 the gravel, or sand, or even the mud on the Channel bottom ? The 

 marine fauna of the district answers this question with an emphatic 

 negative." And again, " The presence of this Molluscan fauna in 

 these very exposed localities is good proof that unchecked tidal 

 currents sweeping over a fairly level sea-bottom are incapable by 

 their own unassisted efforts of raising the sand." 



Now without entering into the discussion of the main question 

 raised by Mr. Eeade, I beg to offer the following observations on the 

 line of evidence suggested by Mr. Hunt, viz. the power of wind 

 waves and tidal currents to disturb the sand or mud of the sea- 

 bottom. To this end, I quote the practical experience of a well- 

 known French marine zoologist, M. Hermann Fol, of the zoological 

 laboratory at Nice, who in his yacht " Amphiaster," was, last 3'ear, 

 entrusted with a mission by the French Minister of Public Instruc- 

 tion, to explore, from a zoological point of view, the littoral of 

 Corsica and Tunis. 



M. Fol is in the frequent habit of donning the diver's dress and 

 descending to depths of from 30 to 100 feet and upwards in search 

 of marine organisms. Quoting from a recent paper (Eev. Sci. June 

 7th, 1890) M. Fol says, " "When there is a swell on the water, the 

 task of the diver becomes very difficult. He is constantly tossed 



