TRANSACTIONS OF SECTION C. 637 



group when the ground was originally mapped. The corresponding rocks of 

 "Wenlock age on the south margin of the Mayo and Galway Silurian basin, near 

 Killary Harbour, are not metamorphosed, and rest unconformably upon the meta- 

 morphic group. 



This stratigraphical break has for many years been supposed to form an 

 insuperable objection to the acceptance of Murchison's conjecture that the meta- 

 morphic rocks of Galway, Mayo, &c., are altered representatives of the Lower 

 Silurian or Ordovician rocks. This, however, is not an obstacle, for a break, 

 accompanied by overfolding and possibly metamorphisra of Lower Silurian strata, 

 has been proved to have occurred in Llandovery times, which admitted of Wenlock 

 or possibly Tarannon beds being unconformable to unmetamorpliosed Lower 

 Silurian, as well as to the metamorpbic group. All this happened prior to a 

 second violent disturbance and overfolding which accompanied the metamorphism 

 of Wenlock strata already mentioned, and which occurred in Ludlow times. 



A comparison of the Lower Silurian series in the west of Ireland with the 

 metamorphic group of the same region and Donegal shows so strong a resemblance 

 between them — as regards the lithological characters of individual members in 

 their original form, their order of succession, and certain peculiar coincidences of 

 associated sedimentary components, described in detail in the paper — that it 

 forms a creditable 7; ?-»na/«e?> argument for their correlation. 



One instance may here be mentioned. At Westport and AchiU Beg thick 

 bands of fine conglomerate, associated with black slate, occur as an integral part 

 of the metamorphic group, while on the south shore of Clew Bay thick bands of 

 fine conglomerate — very similar in character to those in AchiU Beg — occur in 

 association with black slate, which, though sufficiently crushed to justify their 

 inclusion by the original surveyors in the metamorphic ground, are now known 

 to be of Lower Silurian age, identical with rocks of this age in Clare Island. 



The chief objection to ascribing the metamorphic rocks of Mayo and Galway 

 to the Lower Silurian age has been the present difference of condition between 

 them and the fossil- bearing Lower Silurians of the adjoining area. This differ- 

 ence seems to us explicable by conceiving that the great dislocation which occurred 

 in Llandovery times, and occasioned an inversion of strata by overfolding at 

 Salrock between the Killaries, carried unmetamorphosed Ijower Silurian rocks 

 about Leenane against and over rocks of, say, the same age, near Leenane, which 

 had undergone metamorphism in connection with granitic intrusions. These may 

 be seen in the vicinity of Kylemore. Unfortunately the great zone of break is 

 now concealed by newer strata, and further is obscured and complicated by post- 

 Ludlow faults. 



5. Notes on the Irish Primary Bocks, ivith their associated Granitic and 

 Metamorjyhic Bocks. By G. H. Kinahan, M.B.I. A. 



In this communication the writer points out that in previous writings he has 

 insisted that in Ireland there were no Laurentians, because no Irish rocks as a Terrane 

 were similar to the original Laurentians. Now, however, lie has learned that the late 

 Dr. G. M. Dawson and other American geologists class the questionable Grenville 

 series, although in part evidently clastic and volcanic, as Laurentian. Conse- 

 quently, if this is allowed, there are also Laurentians in Ireland and Scotland. 



A short review of the American pre-Palfeozoic rocks and a table of the classi- 

 fications adopted in the United States and in the Dominion are given with Dawson's 

 reasons for his objection to the former, as in it the Animikie and Iluronian 

 are classed together under one title, Algonkian, although there is a profound break 

 between them. Dawson seems to believe the Animikie and the Keweenawan are 

 more allied to the Palaeozoic than to the Archaean : in the latter he would only 

 include the Huronian and the Laurentian. 



A table of the Paltpo^oic rocks, similar to that in the ' Economic Geology of 

 Ireland,' is given and .short descriptions of the different strata. This is succeeded by 

 a general description of the different areas of the Irish pre-Palaeozoic rocks, more 

 especially those of Donegal and Galway. Tliesetwo aretis aro subsequently tabled 



