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The Geognosy and Mineralogy of Scotland

Published online by Cambridge University Press:  14 March 2018

Extract

Having, since file date when the few observations chronicled in a previous number were made, ascertained some more definite facts with regard to these marbles, I now note these.

The localities in which I had previously observed the marble were all (through faulting and intrusions) so obscure as regards its relationships, that any conclusion as to its horizon was not warranted.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1884

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References

page 275 note † In a late discussion at a meeting of the Geological Society (of Edinburgh ?--Ed.) upon this marble (called Dalbeg), it was observed by one of the speakers that it occasionally " becarne eozoie." As this phraseology indicates transition, or gradual passage, it is necessary to remark that it is altogether misleading, so far as I Know of it. Whether the structure be the outcome of the action of the marble upon one includr fragment of gneiss or not, it has to be netted that there is no gradual passing of the one into the other,--no becoming. A line of demarcation, sudden and sharp, is present all round the periphery. Black at once succeeds to white. The thing is, as I stated to the Royal Society of Edinburgh, very like one of the boas worn by our grandmothers, and still liker to one of the dried whale cutlets seen in Farös.

page 276 note * The earbouic acid was uot determined. Doubtless some of the lime and magnesia had been in combination with the soluble silica.

page 281 note * It may not be out of place that I should here state, that after having unravelled the structure of this Scottish eozoon, I thought I was to a certain extent in a favourable position for entering upon an inquiry into the nature of the Canadian, to which it was said to bear so great a resemblance. I cannot say that my specimens of true eozoon are at all good examples, being merely those usually sold; but of these, such as they are, I have just to say, that in them I saw nothing which I did not see in the Scotch; that I did not in them see so much; and that nothing that I did see did I see nearly so well.

page 288 note * "Well seen in the rock of Glen Logan, where the intermixture of the red and white felspar is such that Mr. Budleston pronounced the former to be oligoclase.

page 288 note † " Tackets," anglice "boot-nails."

page 290 note * Mr. Hudleston writes, "I have usually been disposed to regard the abundance of epidosite as one of the features of the " Logan" rock which especially distinguish it from the Hebridean gneiss: but Dr. Heddle states that tiffs peculiar mélange occurs in greater quantity, and perhaps in a purer state in the west of Ross-shire than in any locality known to him. This is in the Hebridean gneiss near Poolewe. Hence this presumed distinction vanishes."

:But Mr. Hudleston here writes in misconception of my meaning. If my description of the Hebridean gneiss be turned to, it will be found that, in Speaking of the Substances occurring therein, I referred to a substance which occurred in "some districts; " which substance I conceived to be the same as one described by Macculloch as found in the western gneiss, and which he thought green felspar. That substance I said I had found in greater quantity at Poolewe than at any other spot where I had got it. My analysis of the Poolewe specimens proved a failure, but enabled me to draw certain conclusions. I knew that Dr. Maceulloch considered it a felspar; I also knew that Mr. l-Iudleston had applied the term epidosite to a similar substance got in the Glen Logan rock, and I wrote, "I conclude that the thing as a whole is that mélange which has been termed epidosite." "I shall, therefore, with a caveat term it mean-while epidosite." I also stated or indicated the reasons for the caveat, namely, that both the Loch Marce and the Harris specimens showed incipient crystallisations cf recognisable epidote.

Now this may not be distinctive between the substances occurring in the two rocks; but so far as my experience goes the substance from the Hebridean alwayss shows incipient crystallisation; while in no one of the many slices which I possess of that occurring in the Logan rock, have I ever seen a trace of this. In all the slices the green matter passes into and through qual'tz, like the fibres of an untwisted cord. or as a cloudy stain. Be this as it may, instead of occurring as do the veins of the substance found in the lower rock, only in some dislricts, epidosite occurs everywhere throughout the Logan rock; and though I have not yet found it in perhaps quite such thick veins as those in which MaccullJch's " felspar-like mineral' is found at Poolewe, yet it is present in certainly a very much larger total amount. I therefore go quite along with Mr. Hudleston in being " disposed to regard tbe abundance of his epidosite as one of the features of the Logan rock, which especially distinguishes "it from the Hebridean gneiss." Hence Mr. Hudleston'a "presumed distinction" does not vanish, but it is fortified.

page 298 note * And- others in Cromarty.

page 301 note * Sir Robert Christison found apophyllite in a Coal-measure limestone quarry, at Chapel in Fifeshire; but it was in a cavity, and evidently tlad been relined through water infiltration,

page 306 note * The same igneous rock cuts "Logan" on the north side of Coneveall.

page 316 note * See pp. 241,242.

page note * Dr. Charles Rickets, Mr. Starkie Gardner, and the Rev, Osmond Fisher.

page note † Mr. T. F.-Jameson and Dr. Rickets.

page note * I have qualitatively and to a certain extent quantitatively, examined specimens from each of the above rocks with a view of seeing if they can by analysis be distinguished from tile dolomites of the west. I found them to be all more or less dolomitie, and they can be readily distingtJished by the presence of alumina and of phosphates. Taken as a whole they are not nearly so pure as is the rock at Assynt.