1. Summary of the Evidence regarding the Primary Glaciation.—

We must now, very briefly, recapitulate the evidence regarding the primary glaciation of Shetland, in order to show the conclusions which may be justly drawn from the facts, and also to determine the relation which the glacial phenomena of these isles bear to the glaciation of Norway and Scotland.

It has been shown that in the islands of Unst, Fetlar, Whalsey, the Outskerries, Bressay, and along the eastern sea-board of the Mainland and Yell, there is one uniform system of ice-markings trending W.S.W., S.W., and in some cases S.S.W.; while in the western districts of the two latter islands, as well as in Meikle Rooe, Papa Stour, and Foula, the striæ swing round to the N.W. and N.N. W. From a careful examination of the striated surfaces and the Stossseite of the roches moutonnées, it is evident that the agent which produced them must have crossed the islands from the North Sea to the Atlantic. Fortunately this conclusion is page 807 placed beyond all doubt by the distribution of the Boulder-clay, as well as by the dispersal of the stones in this deposit. On the western sea-board of Unst the Boulder-clay contains fragments of serpentine, gabbro, and graphitic schists, all of which occur in situ on the east side of the Vallafield range. Moreover the relative distribution of the serpentine and gabbro stones in this deposit on the western shore is in direct proportion to the relative areas occupied by these rocks to the east of the watershed. It follows, therefore, that the agent which glaciated Unst must have crossed the watershed, carrying the bottom-moraine up the slope, and depositing it in the lee of the range. In Fetlar, blocks of gabbro and serpeutine are likewise found in the Bouldor-clay on the west coast; while along the east coast of Yell, blocks of gabbro occur in this deposit which have been brought from Unst and Fetlar, testifying alike to the same westerly movement.

The evidence derived from an examination of the Boulder-clay sections on the Mainland is equally conclusive; for it matters not whether we cross the northern, central, or southern portions of the island, we are compelled to admit that the ice-flow during the primary glaciation must have been towards the Atlantic. In the central part of Northmavine it has been clearly proved that the Boulder-clay partakes of the physical character of the rock-formation on which it rests, while a certain percentage of the stones is derived from localities which lay in the path of the glaciating agent. Abundant evidence has been adduced to show that the quartz-felsite area between Tanwick and Roeness Voe has been invaded by the diorite stones, while the area occupied by the bedded porphyrites has been invaded by the quartz-felsito and diorito stones. Moreover it is particularly observable that the blocks do-rived from the successive areas occupied by these rocks, which are present in the Boulder-clay, diminish in number in proportion to the distance from their parent source.

Again, in the long tongue of land which stretches from Scalloway southwards to Fitful Head, blocks of the Old-Bed-Sandstone rocks occurring on the eastern sea-board are found, not only on the tops of the highest hills, but also in the Boulder-clay on the western shore. Moreover the distribution of the fragments of the Lerwick Sandstone, Brenista Flags, and basement-breccia in the Boulder-clay north of West Quarff is in perfect harmony with the relative areas occupied by these subdivisions of the Old Bed Sandstone south of Lerwick. The same relationship holds true in the district between Maywick and Fitful Head. It is clear, therefore, that the glaciating agent must have overflowed the watershed, as we found to be the case in Unst.