416 MR. G. BARROW OX THE MOINE GNEISSES [Nov. 1964, 



staurolite : but on either side of the belt the metamorphism remains 

 singularly constant over very large areas. 



In the Tilt Valley, as the gneisses are followed, they lose rapidly 

 their crystalline character. But, in addition to this, they also thin 

 away and change in composition, passing into a thin gronp of 

 equally parallel-banded rocks, known locally as ' the Honestoues.' 



To establish clearly and fully these changes in the aspect of the 

 Moine Gneisses, it was necessary to find a more or less continuous 

 section that should at once show both the decreasing crystallization 

 and the decreasing thickness. Such a section has been found in the 

 Tilt Valley, in the neighbourhood of Gilbert's Bridge, where the river, 

 instead of continuing its usual rather straight course, makes a big 

 bend toward the north-west. Now the arch of this bend is at one- 

 side of the belt of decreasing metamorphism and thickness, while 

 the two ends are at the other ; and it is by means of this section 

 that the whole explanation was ultimately arrived at. 



• 



(b) The Section at Gilbert's Bridge. 



(Ma P? PL XXXIII & fig. 9, p. 444.) 



Standing on Gilbert's Bridge, and looking up the Tilt, we see 

 a typical section of the parallel-banded gneissose flagstones or 

 Moine Gneisses, striking up-stream, and having an even dip to the 

 south-east, at about 30°. Below the Bridge the same rocks are seen 

 for a few yards, but farther down only small patches of gneiss are 

 exposed, as the rest of the section is composed of numerous infolds 

 of other beds. An examination of the river-channel shows that 

 these rocks are intensely folded, one of them, consisting of limestone, 

 being repeated no less than eight times in a distance of 150 yards. 

 This bed may be either in contact with the Moine Gneiss (here, at 

 times, highly quartzose) or separated by one or all of three beds or 

 bands. Of these, the most striking is a curious 'pink rock," of 

 which the dominant constituent is obviously felspar. Another is a 

 dark and often tough schist, which varies somewhat in aspect ; 

 while the third is a little sill of hornblende-schist, which, by folding 

 on itself, may attain a thickness of more than 6 feet, but is some- 

 times not seen owing to a slight change of horizon, one of its 

 characteristic features. This little sill is of considerable importance, 

 as fixing, approximately, the horizon with which we are dealing. 



The ' Pink Felspathic Rock,' or rather material (for it is found in 

 the other sedimentary rocks) consists mainly of microcline. Where 

 purest, it forms a separate band, which, a little below Gilbert's 

 Bridge, attains a thickness of 6 feet, owing, partly, to repetition by 

 folding. Some way below the Bridge it occurs as lenticles in the 

 quartzose rock. It is evenly disseminated through part of the 

 limestone in one place, while at another it forms a segregated patch, 

 which has so completely recrystallized as to simulate a pegmatite, 

 a mode of occurrence that has been noted over a wide area. A 

 specimen from the thicker band (10,534) is composed of abundant 

 microcline, quartz, decomposed felspar, and a considerable quantity 

 of green and brown mica, with a parallel arrangement. In some 



