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XV. The Geognosy and Mineralogy of Scotland

Published online by Cambridge University Press:  14 March 2018

Heddle
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Extract

Anyone who contemplates on a map the great size of this island in geographic extension, and its extraordinarily irregular outline, would conclude that it must be composed of various formations, and that the connections and dispositions of these must be intricate in the extreme.

The latter conclusion will, however, upon a more careful observation, be found to apply only to the western portion of the island. However deeply trenched by the sea iu its lower grounds, however persistently thrusting its cliffs oceanward in its higher, the ridged and furrowed outlines of the eastern, disposed in almost rectilinear sequence, indicate but one formation,—various as the members of that formation may in mere lithological features appear.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 2 , Issue 11 , December 1878 , pp. 155 - 190
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1878

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References

Page 156 Note * East of Lerwick a quarry of dark-grey flagstones, which are profusely sprinkled with mica, contains a peculiar calamite-like organism of sometimes a couple of feet in length ; this has a bulbous dilatation with apparently rootlets. Specimens of small fishes, apparently acanthoides, were shown the writer ; these were imbedded in a brown finegrained muddy sandstone ; they ware stated to occur in a quarry north of Gardie, in Bressay.

Page 159 Note * Time did not suffice for my endeavouring to attain to the cause of this anomalous endurance.

Page 163 Note * And those of many who it is to be suspected never were there. A fervently constitutional conservative gentleman, who was once on the spot with the writer, availed himself of this licence, by engraving the name of the reigning monarch in characters of loyally surpassing depth.

Page 163 Note † e.g In Jukes' Geology, p. 83, we read "Potstone includes the impurer, granular, dark-coloured varieties of steatite ;" p. 143 we read " Potstone is a massive form of chlorite schist," In Cotta, p. 251, we read "Potstone consists of a felt-like web of chlorite ; sp. gr. 2.8 (?) ; content of silica, 30—60 p. c. (?)" How long are geologists to be content with such a knowledge of the composition of rocks as is expressed by " 30—60 p. c. (?)" ? The ouly full and correct description of potstone which I can find is in Haidinger's translation of Mobs' mineralogy.

Page 165 Note * I have almost invariably acted upon the principle of sacrificing even a unique specimen of an unknown mineral for analysis, should there be a sufficiency for that purpose, —there was not so here.

Page 165 Note † A substance almost indistinguishable in appearance occurs in veins cutting a gabbro on the east side of Beauty Hill, this proved to be pseudophite : this was evidently formed by a change in the labradorite.

Page 170 Note * Smaragdite and a, to me unknown—striated red mineral are also visible in the microscope.

Page 173 Note * I am indebted to my friend Dr. Gordon, of Birnie, for enabling me to furnish a minute description of the rocks of the north shore lying eastward of the breecia ; my time when walking across the section permitting only of my noting the sequence, but not of my gathering specimens of the rocks.

Dr. Gordon, who during several visits to Shetland, had been engaged in independent geological research, kindly went over from Hillswick and took specimens along this north shore, and elsewhere in the district.

Though from the great amount of mineral analysis which I have yet before me, I am at present unable to analyse these rocks. I yet consider several of the specimens collected to be so typical examples of rocks of which we have no analyses confirmatory of the names which have been assigned to them, that I still hope to be able to supply this want. And if so, I take this opportunity of stating that Dr. Gorden purposes to deposit the specimens analysed in the Elgin museum.

Page 177 Note * I should add in connection with this series that among the rocks forwarded to me by Dr. Gordon, there are two which I and my confrere Mr. Dudgeon did not observe : 1st, a talc rock, which was interesting and typical. It seemed to consist of plates of pale green talc, with a very little actynolite ; these are twisted into one another in a singular manner. The knife also suspects the presence of granules of quartz. 2nd, a crystalline granular variety of the beautiful rock (﹖) containing specks of pyrite ; this is very like an igneons rock in structure. I have seen large rolled masses of this rock near Hillswick.

Page 183 Note * Under the microscope albite has an appearance, by polarised light, which is quite distinctive ; the optical characters of this and other felspars will bc considered apart.

Page 190 Note * The vacuity left by the washed out dyke at the Cannon, has two openings—one which is constantly beneath the waters, and is unseen ; another circular, and about a yard in width, is a little above water level. The inner chamber is of such profundity and form, that at certain states of the tide, the included air pent up by an inrolling surge, asserts its elasticity upon the compressing liquid in its back sweep, in such a way as to catch some of its upper waters and discharge them, dissipated into steam This is accomplished with terrific force, and a noise which, to a near observer, resembles the imagined snort of a leviathan ; while when heard from a passing vessel the reverberation seems like the sound of a cannon. The misty spray is discharged from the throat of the recess with astounding force to a distance of forty yards.

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