Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/166



By, M.D. F.L.S. President of the Geological Society, Chemist to the ordnance, Lecturer on Chemistry at the Royal Military Academy, and Geologist to the Trigonometrical Survey.

the Sketch of the Mineralogy of Sky was drawn up for the Society's volume in 1813, The differences between the map of Sky which accompanies this supplementary paper, and that formerly given, require some explanation, that the readers of the Geological Transactions may be able to appreciate the reasons which have led to the alterations, and account for those which, in a geological point of view, will be shown to be more apparent than real, and to see what degree of confidence they may repose in the present one.

In laying down the places of the different rocks, I formerly made use of the documents from which Mr. Faden's travelling map was constructed, and I was guided in the examination by Mackenzie's chart. The incorrectness of these has been the leading cause of the apparent differences in the relative extent of the rocks as they have been delineated, while their insufficiency for the purposes of a reticulum, arising from the great distortions of the outline of the country and their deficiencies in its internal features, prevented me from laying down many of the places where the existence or boundaries of particular rocks had been accurately ascertained. Hence the geological delineation bore no proportion to the accuracy of the observations. To these defects if there be added the blanks which the partial nature of the former survey necessarily left, the discrepancies of the two will not appear so considerable as they seem at first sight. I may also add, that it has been found expedient to change the colours by which the different rocks are denoted; another apparent cause of difference.

For the present observations I have made use of Mr. Arrowsmith's map, with certain variations which I think it necessary to explain, that the degree of confidence to be reposed, whether in the original or in the alterations, may be understood.

A line drawn from the head of Loch Snizort to that of Loch Slapin will, under slight exceptions, separate the estate of Lord Macdonald from those which lie to the westward of it. It is not too much to say of the latter division that the outline of the coast is incorrect, and the internal features something worse than conjectural. But the division which constitutes Lord Macdonald's estate, having been taken from a survey of that district executed by an intelligent provincial surveyor for his Lordship, is deserving of considerable confidence, and will be found over most parts sufficiently accurate in its internal features for the purposes of the geological record. Even in the latter I have found it necessary to make some additions and alterations; in the former part of the island these are considerable. I must however say, that even in this case I have limited the corrections to those parts of the map where it was necessary to denote the boundaries of different rocks; the northern parts, presenting only one substance, were not in immediate need of it, and I do not pretend to arrange the geography of the island. It is necessary to point out the alterations which have been made.

The region lying between Loch Eishort and the foot of the syenite hills, which includes the valley of Strath, has been redrawn and reduced as far as was possible to the present size, the intricacy of the position in the rocks which constitute it absolutely requiring a more accurate detail of the leading features of the ground. Slight alterations have been made in the shades which indicate the relative altitudes of the hills, wherever that was called for, and the result will be apparent to those who shall compare the present map with Arrowsmith's. At the point of Sleat and in the Kyle ri'ch two or three alterations have been made in the line of coast, these being absolutely required; others which might have been made have been omitted, as they were not wanted for the particular objects of this work.

The changes made on the eastern side are much more important. The estate of Strathaird which includes Blaven and Coruisk, has been corrected from a private survey in possession of the proprietor, Mr. Macalister. The Cuchullin hills are in the original map entirely misplaced, while the remarkable lake above mentioned has been omitted. These I have rectified as far as was in my power, since both their form and extent were important. The tract to the north of this including Mr. Macleod's property, remains with very little alteration, since its uniformity of composition did not call for any particular care.

The directions of the strata are marked by prolonged lines extending to the neighbouring shores or islands wherever they presented the same substances, and on these the tendency as well as the quantity of the dip is occasionally pointed out by an appropriate index and numbers, wherever it seemed necessary to specify them.

Finally, whatever corrections may be found in this map are merely intended to serve the purpose of this paper; the formation of a correct map of Sky must yet be considered a work far distant. I had no prospect of again visiting that remote country. Circumstances having led me to traverse the