GLACIATION OF THE SHETLAND ISLES. 795 
gabbro and serpentine areas. Due west of the Sound, in the hollow 
along which flows the Baliasta burn, there is a considerable covering 
of Boulder-clay, the included stones being mainly composed of ser- 
pentine and dark graphitic schist, though the underlying rock con- 
sists of green chloritic schist and gneiss. Ascending the Vallafield 
ridge, the slope is found to be covered with heather and peat, and 
well-nigh destitute of drift. Where this covering has been removed, 
numerous bleached fragments of serpentine are to be found ; while 
near the top of the ridge, where the slope is more gentle, occasional 
patches of Boulder-clay are met with in which well-striated frag- 
ments of serpentine, gabbro, and black schist occur. No Boulder- 
clay is to be seen on the watershed, which reaches a height of over 
600 feet at this point ; still, where the peat is worn away, a few 
bleached fragments of serpentine are observable. 
Along the western coast, from Woodwick to Wick Bay, a narrow 
ledge or terrace intervenes between the rock-slope and the coast- 
line, which is covered with Boulder-clay more or less continuously. 
Excellent sections of it are exposed at the heads of the numerous 
geos. At Collaster it consists of a tough fawn-coloured clay, full of 
striated stones of all sizes up to blocks 2 to 3 feet long. The fol- 
lowing percentages were taken from the banks of the voe at this 
locality :— 
North side South side 
of bay. of bay. 
Gneiss and schist (underlying rock).... 53 per cent. 60 per cent. 
Serpentine (from east side of watershed) 32 a 22 9 
Gabbro 09 » 0 uh 0» 9 9 
Black schist is st iM i te 3 ie 
AY SIT HUET A Nee ean Senet ao eee 2 AS 3 ‘ 
Granite (from Lambaness) .......... il as 2 5 
NCU OT ATULC y./0 8. snc /6 donc low Paes CaN Me 1 nf 
100 100 
Tn all the sections south of Collaster, towards Wick Bay, fragments 
of serpentine and gabbro are invariably present in this deposit. 
Moreover, it is important to note that the relative distribution 
of the gabbro and serpentine stones in the Boulder-clay between 
these localities is in direct proportion to the respective areas occupied 
by these rocks on the east side of the watershed. The following are 
the proportions in the Boulder-clay sections at three localities :— 
Serpentine. Gabbro. 
North side of Collaster Voe .......... 32 per cent. 11 per cent. 
HENIILOMMINGRR rie sie rr cine sccie vce aiens« 2A “ 22 be 
‘iid a Say om RRR Sree GRRE eee Oa 22 re 26 - 
This relative distribution of the stones is not a mere accident ; 
for a glance at the map will show that to the E. and E.N.E. of Col- 
laster the serpentine occupies a much greater broadth of ground than 
the gabbro, while to the east of Wick Bay the conditions are 
reversed. Such a direct relationship is inexplicable on the hypo- 
