r 283 ] 



XXVII. 



ON GEOLOGICAL UNCONFOEMABILITIES. By G. H. 

 KINAHAN, M.B.I.A., Etc. 



[Read January 9, 1889.] 



There appears to be a very general misunderstanding as to the 

 ordinary character of an unconforrn ability between two geological 

 formations. In a simple unconformability the later rocks lie on the 

 upturned edges of the older ; but as far as my experience goes, 

 such simple unconformabilities, especially among the lower 

 Palaeozoic formations, are the exception and not the rule ; asj in 

 most cases of junction now exposed, the later rocks often accumu- 

 lated, as much against, as on the older rocks. In such cases the 

 older and later rocks may strike and dip nearly similarly. Fortu- 

 nately, in Ireland, when such are their relations, one set is gene- 

 rally more or less metamorphosed, while the other is not so, and 

 under such circumstances the boundary can be drawn by the litho- 

 logical distinctive characters ; but if neither one nor the other is 

 altered, or if both are altered, it is often very difficult to say where 

 the exact boundary is. I have found that marked distinctions in 

 groups of rocks usually leads an observer to overlook their relative 

 positions ; and it is not until the groups display very similar 

 characters that he looks back on other places and learns that, 

 except for the distinct mineral arrangements, the mapping of the 

 rest of the boundaries would not have been as easy as he had 

 found it. 



Among the older rocks, to the want of a simple unconforma- 

 bility, is to be added the complications due to foldings, contortions, 

 faults, and thrusts (reverse faults), all of which add to the intri- 

 cacies of an unconformable boundary ; more especially so if both 

 the older and later rocks have been metamorphosed. 



My difficulties, I presume, have been felt by others ; it may, 

 therefore, be useful to lay before the Society some general notes 



