KiNAHAN — On Canadian^ Arc/man, and Pre- Cambrian Bocks. 353 



In the counties Galway and Mayo different phases of meta- 

 morphism can be studied. At first everything may appear to be 

 under general law, but when details are entered into, here, as 

 elsewhere, there are peculiarities hard to be explained. 



Gro northward from the ground westward of Gralway town to 

 Oughterard, across the strike of the rocks, and you first meet 

 porphyritic oligoclasic granite, which changes nearly impercep- 

 tibly into oligoclasic porphyritic gneissic granite ; while the latter 

 graduates through gneiss into schist, and still further north, in 

 the county Mayo, into unaltered Cambro-Silurians. Although in 

 general the gneissic granite graduates into gneiss, yet in some 

 places there is a hard boundary between them. 



Go westward along the strike of the rocks and the change is 

 not so gradual, as in places there are sudden jumps from granitic 

 to schistose rocks ; while in the latter you find outliers, sometimes 

 having a nucleus of porphyritic oligoclasic granite, but more 

 often porphyritic gneissic granite, which are more or less suddenly 

 replaced by schist. 



If, however, we go southward, there is a complete change in 

 the character of the metamorphism;. as in places along Gralway 

 Bay, margining the porphyritic oligoclasic granite, there are 

 gneissic granites, usually orthoclasie and fine-grained, but having 

 in them coarse-grained and oligoclasic beds; also to the S. E., at 

 Gralway town, there is a completely different group of rocks ; as 

 suddenly, adjoining the granite, and in a few small patches on it, 

 or entangled in it, are schists principally hornblendytes. This 

 small area of schists, apparently in such close proximity to the 

 mass of the oligoclasic porphyritic granite seems very peculiar; 

 this might, however, be explained by referring to phenomena that 

 can be studied farther westward, near Eoundstone and Slyne 

 Head. As, however, it would take some time to do so, and the sub- 

 ject is in part foreign to the present inquiry, it seems sufficient to 

 notice it without going into further particulars. 



In the county Gralway the coarse oligoclasic gneissic granites 

 are evidently, especially across the strike, a portion of the series of 

 gradation from metamorphic granite to schist and unaltered rocks ; 

 but in the Oastlebar district, county Mayo, as first pointed out by 

 Symes, there are undoubted intrusive masses or courses of a simi- 

 lar rock. These Mayo rocks seem interesting, as in the Canadian 



