KiNAHAN — On Gfanitic and other Inijenite Rucks. 135 



Either of these kinds of granite woukl, at any time prior to their 

 final solidification, be liable to forra intrusive masses or veins, being 

 forced up into fissures, cracks, or any other kind of " shrinkage fissure," 

 or vacancy formed during the cooling of the overlying rocks, the 

 resulting rocks being respectively oligoclasic or orthoclasic granite, 

 according to the magma from whence they came. Furthermore, after 

 the mass of either the oligoclasic or orthoclasic granite had cooled, 

 there would still be centres containing unsolidified portions, that at any 

 time would be liable to be forced up into the shrinkage fissures as the mass 

 cooled. The veins, Granityte, thus formed would necessarily be more sili- 

 ceous than the associated rock-mass ; but in the oligoclasic granite, 

 although clifi'erent from the associated rock, they might be nearly similar, 

 and scarcely distinguishable from the orthoclasic granite ; however, such 

 veins in the orthoclasic granite would be quite distinct, being more 

 siliceous than the mass. In this way a solution for the formation of 

 the quartzitic granite might be suggested ; for, if we could imagine a 

 fluid centre becoming more and more siliceous as zone after zone 

 cooled, the residue finally might be so siliceous as to be similar, or 

 nearly similar, in composition to quartzyte. 



If the granitic and other ingenite rocks were formed under circum- 

 stances similar to those above suggested, all the relations between the 

 different rocks could be easily explained. In West Galway, and 

 South-west Mayo, these all graduate through other rocks into each 

 individual rock, yet none of them pass suddenly into a different class 

 rock — each must graduate through the intervening passage-rocks. A 

 schist must graduate through a gneiss into a non-intrusive oligoclasic 

 gTanite ; a gneiss through a non-intrusive oligoclasic granite into an 

 intrusive oligoclasic granite — a non-intrusive oligoclasic granite tlirough 

 an intrusive oligoclasic granite into an intrusive orthoclasic granite, 

 and the latter rock through an elvanyte into a plutonic rock, while 

 among the latter class of rocks a petrosilex must graduate through an 

 euryte into a whinstone ; all the other graduations are similar, there- 

 fore, it appears unnecessary to enumerate them, and such changes are 

 all accounted for by the above suggestions.* The volcanic and phdonic 

 rocks would have to range fi^om basic to highly-siliceous, according to 

 the zone and time fi'om and at which they were intruded ; so would 

 the elvanytes, and so would the intrusive granites. The granite formed 

 in situ would be very varied in its composition, on account of the 

 various rocks from which it originated ; yet it would graduate into 

 intrusive granite, and so form one of the connecting links between the 

 plutonic and the metamorphic sedimentary rocks. 



The plutonic and volcanic rocks, being surface-formed, should only 

 be found among surface-rocks; but as all may at sometime or other be 

 at the surface of the earth, they are found breaking up tlirough all 



COlllltl 



In another place a list is given of the changes renuirked in the rocks of this 

 ry. — Journal Roy. Geol. Soc. Ireland, voL iii., 1871, p. 8. 



