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X. 



THE SILURIAN AND META^IORPHIC EOCKS OF MAYO 

 AND NORTH GALWAY. 



By J. R. KILROE. 



[cOlIiirXICATED WITH THE PEEillSSIOX OF THE DIRECTOE OE THE 

 GEOLOGICAL SURVEY OE IRELA.VD.j 



Plates IX, X. 



Eead June 24. Ordered for Publication June 26. 

 Published September 28, 1907. 



It is usually gratifying to consider the way-marks which line the 

 path of geological progress, hut not always a pleasing experience 

 to read past conclusions in the light of present knowledge. 

 Hypotheses which temporarily serve useful ends are apt to become 

 stereotyped in the hands of a few, and in this form tend to hinder 

 rather than aid inquiry. Geological maps, which can only aim at 

 presenting summaries of facts, necessarily display a colouring of 

 the views according to which the facts were grouped ; and for this 

 reason they are liable to share the fate of superseded hypotheses. 

 In so far, however, as they afford guidance to points of geological 

 evidence, they are always valuable. 



The area here described extends from the north coast of Mayo to 

 the Maamturk range in Galway, and comprises the areas repre- 

 sented on the published 1-inch sheets of the Geological Survey 

 of Ireland, numbers 39, 40, 51, 52, 62, 63, 73, 74, 75, 83, 84, 

 85, 93, 94, 95, issued in the years 1869 to 1879; with accompany- 

 ing Memoirs, the last of which was issued in 1881. It would be 

 odd indeed if a quarter of a century's geological research at home 

 and abroad did not beget such increase of light, and changes in 

 point of view, as demand a revision alike of the evidence and 

 conclusions upon the strength of which the mapping was originally 

 planned. This will best be understood when the nature and bearing 

 of some factors in the case are here stated ; and first as regards the 

 Silurian strata, thus : — 



1. Fossils occurring usually in, and by some even at the time 

 believed to be fairly determinative of, Caradoc rocks, hud been 



R.I. A. PKOC, VOL. XXVI., SEC. B. F 



