46 Correspondence — Mr. G. H. Kinahan. 



to bring to bear on that side of the question. But I have thought 

 it worth while to recall attention to the Lexden subsidence, and to 

 my remarks upon it. 



0. Fisher. 

 Harlton, Cambridge, 11 Bee. 1881. 



JUKES AND THE SUPPOSED LAURENTIAN EOCKS IN DONEGAL, 



IRELAND. 



Sir, — From a letter that I have received, it would appear that 

 some question my statement in the letter on the " Lower Palaeozoic 

 Rocks of Wexford," that Juices first suggested the possibility of there 

 being Laurentian roclcs in Donegal. I find that new men in new 

 countries, who do not take the trouble to learn what others have 

 previously done, often bring forward " new discoveries," which, 

 although new to them, are well known and old to those acquainted 

 with the country. The present question appears to be a case in 

 point. It must be about twenty years ago when Jukes first sug- 

 gested that some of the rocks in Donegal were possibly of Lauren- 

 tian age, and when King, of Galway, made a similar statement in 

 reference to the Connemara rocks. At all events, when I was sent 

 to the West Galway district about the year 1863, I was specially 

 instructed on this point, because it was supposed that possibly 

 Laurentian rocks might be found in Galway, Mayo, and Donegal. 

 While working in North-west Gonnaught from 1863 to 1871, 1 have 

 over and over again discussed the probability of Laurentian existing 

 in the above-named counties with Jukes, King, Melville, Harkness, 

 and all other geologists who visited me during those years, among 

 whom was Prof. Hull. I suspect that even Sir R. I. Murchison, in 

 th? papers published in the Geol. Mag. about that time, also mentions 

 Mr. Jukes' suggestion as to the Laurentian age of some of the 

 Donegal rock, but I cannot here refer to those papers. I therefore 

 believe that I am quite justified in stating as I have done in the fii'st 

 chapter of the " Geology of Ireland," and in mj late letter to the 

 Geol. Mag., that any credit due is due to Prof. Jukes, until some 

 one works out the question in detail ; which has still to be done. 

 At present even the age of the associated rocks with those suggested 

 to be Laurentians is uncertain. They may be of the same age as 

 those at Creggaunbaun, south of Clew Bay, which have been proved 

 by Sj'me to be Upper Silurians ; or they may be Cambro-Siluriaus ; 

 or in part they may be, as suggested by me in the paper read before 

 the Royal Irish Academy, " On supposed Cambrians in Cos. Tyrone 

 and Mayo," Cambrians ; or, as does not appear improbable, if the 

 statements made in reference to the Donegal rocks are correct, all the 

 rocks of the country may belong to one sequence, the supposed 

 Laurentians being a portion that is more metamorphosed than the 

 rest ; and as in many other metamorphic regions, brought down by 

 a fault or faults into a juxtaposition with less altered rocks. 



The latter suggestion is a very natural one, when we consider 

 that on account of a similar position of rocks, a portion of the rocks 

 of West Galway are said to be of Laurentian age. Now, in West 



