KiNAHAN — Metaniorphic Action. 473 



metamorphic-granite, with a few subordinate schistose beds and 

 sometimes limestone ; sixth, gneissic- granite (that is, a rode that, 

 although foliated, partakes more of the granite than gneiss characters), 

 with alternations of schist ; seventh, schist and limestone. It seems 

 remarkable that where sections occur, showing the rocks at each side 

 of a mass of metamorphic-granite, both sections are rarely similar ; as 

 on one side the gradation may be gradual, while on the other side it is 

 abrupt. This is the case in Galway, Mayo, Sligo (?), and Donegal : 

 in the last there is a schistose zone in IS'o. 5. The gradual and 

 abrupt gradations seem, in most cases, to be due principally to the 

 chemical composition and physical properties of the original rocks, as 

 some are much more susceptible of sudden change than others. 



Intrusions of granite, felstone, whinstone or allied rocks, at their 

 genesis, must have had more or less well-defined boundaries ; and, 

 although younger than the rocks in which they occur as intrudes, 

 may, in a great measure, underlie them.^ If such intrudes are subse- 

 quently metamorphosed, but at the same time as the associated rocks, 

 they necessarily will retain the original hard boundaries ; which 

 probably may become even more marked, as the rocks of the intrude will 

 change into a rock more or less different from those into which the 

 associated sedimentary rocks are changed ; as all eruptive rocks in 

 the early stages of metamorphism are more prone to change into some of the 

 varieties of gneiss or granitic rocks than into varieties of schist ; while 

 most sedimentary rocks, if subjected to a similar degree of metamorphic 

 action, ivill he changed into schists. 



Such very different results of metamorphic action, which can be 

 seen in nearly every tract of metamorphic rocks, let them be small or 

 large, seem to be a stumbling-block to many observers, who will 

 insist that such tracts of more conspicuously developed rocks, on 

 account of their distinct character and hard boundaries, must be older 

 than the associated rocks. 



Apparently it is an anomaly to suggest that any gneiss could 

 originally have been granite. Yet that such is the case can be proved 

 even in a small island like Ireland. To give two examples : — The 

 Slieve Croob granite, county Down, was intruded into the Cambro- 

 Silurian ; and although in general a mass, yet from it, in places, 

 there were branches, principally veins of four or five inches to many 

 feet wide. Subsequently, a tract of country along the line of the 

 margin of the granite intrude (including a strip of the granite and 

 a strip of the adjoining rocks) was subjected to an invasion of meta- 

 moi'phic action, which changed the granite affected into gneiss, while 



" SucIl intrudes must not be confounded with pre-existing bills ox protrudes ; 

 as the latter, although made up of rocks very similar in general character, existed 

 before the sedimentary rocks were deposited, the latter accumulating around and on 

 them. Intrudes and protrudes, in general, can be easily distinguished, as the 

 former nearly invariably sends courses, branches, or veins into the associntetl sedi- 

 mentary rocks, while a protrude does not, it often having a conglomeritic or 

 brecciated rock in part margining it. 



