156 , The hHsh Naturalisf. 



A little beyond this cutting, a dyke of compact basalt about 

 one foot wide was cut through in making an outlet for the 

 drains. Northward from the granite cutting the Maam Valley 

 fault may be traced to I^ough Corrib, its course being marked 

 by great masses of crushed quartzite. In one place a newer 

 dyke of intrusive rock cuts across the fault ; Professor Cole 

 regards this as a quartz-aphanite. A basalt dyke seems to 

 have come up alongside and mingled with the aphanite. 



Near the basalt dyke in the outlet before described, a large 

 boss of schist has been cleared of peat, and on the fresh 

 surface may be seen a network of granite veins, which 

 suggests that the granite was intruded in a very liquid 

 condition. The line now enters the bog, through which it 

 runs for nearly four miles, with the exception of some cutting 

 in rock, which often rises in bosses above the surface. In 

 excavating the foundations for Maam Cross station, limestone 

 was met with under the peat, and this rock may also be seen 

 west of the road at the station. North-east of Lough Shindilla 

 some schists, much cut up by granite, may be seen on the 

 right. The schists contain much quartz, probably deposited 

 from hot waters. 



Cutting North of Lough Shindii,i,a. 

 At the east end are greenish and purplish quartzitic mica 

 schists. On the purple foliation planes are often pale-green 

 bands about \ inch wide, straight and fairly parallel, and cut 

 by similar systems of bands. These seem to be due to de- 

 composition, with production of chlorite, along very fine 

 joints. Near the centre of the cutting are quartzitic schists 

 that contain carbonates, probably calcite, in the mass, and 

 more abundantly on the joint-planes. Farther on, the schist 

 approaches the character of a granulite. The schists in this 

 cutting appear in great part to occur in regular beds, and 

 might possibly be metamorphosed sedimentary rocks. There 

 are many granite veins in the cutting ; some are narrow, with 

 straight parallel walls, and they are generally vertical. 

 . These veins were probably formed by the granite filling old 

 joints. The granite in them is father coarse-grained ; the 

 mica flakes are generally most plentiful in the centre, but are 

 sometimes arranged in two zones parallel to the walls. There 

 are other larger and more irregular veins, often with ill- 

 defined edges, as if the schist and granite had fused together. 



