of some of them (that extraordinary phenomenon which has justly excited the wonder and severely taxed the ingenuity of Murchison and other geologists who have examined and attempted to account for it) might be due to the breaking-up and redeposition of some of the beds*.
I shall now proceed to describe the facts which I have been able to observe concerning the position of tbe Jurassic strata, and their relations to the Palaeozoic rocks, and then indicate the conclusions to which they point as to the circumstances of deposition and of the subsequent disturbances of these strata. By this means I hope to be able to demonstrate what were the remote causes which led to the preservation of the interesting masses of Mesozoic strata to be described in the present memoir.
It will be convenient, in order to make the subject as clear as possible, to describe a line of section passing across the centre of the most important Jurassic area, where the rocks attain their greatest development and present the most satisfactory exposures. Such a line of section we have, as pointed out by Sir Roderick Murchison, passing through Clyne and Brora, and crossing a breadth of upwards of two miles of Secondary strata. When the main facts with regard to this most important section have been established, it will only be necessary to refer, in more general terms, to the various sections to the north and south of it respectively which serve to illustrate the relations of the Secondary to the Primary strata.
§ 1. Description of the Section through Beinn-Smeorail, Clyne Kirk, and Brora, N.W. to S.E. (fig. 1).
The great series of metamorphic rocks which covers all the central parts of Sutherland, have now, through the discovery of fossils by Mr. C. Peach, and the study of the physical relations of the beds by Sir Roderick Murchison and Professors Ramsay, Geikie, and Harkness, been referred to the age of the Lower Silurian†. These rocks, which have been very happily termed by Murchison " altered flagstones," present the most varied characters, passing from flaggy quartzites or altered sandstones, in which crystalline minerals just begin to appear along the planes of stratification, up to the most highly granitic gneiss, and perhaps into true granites. The prevailing dip of these strata is towards the south-east ; and they are usually inclined at very high angles, often greatly contorted, and sometimes traversed by numerous veins of granite, quartz, felspar, &c. As a rule they do not form striking elevations, and give rise to but tame and monotonous scenery‡.
- "On the Geognosy of Sutherlandshire," by R. J. H. Cunningham (1839),
p. 37, published in vol. xiii. of the ' Transactions of the Highland and Agricultural Society of Scotland.'
† Quart. Journ. Geol. Soc. vol. xiv. (1858) p. 501 ; vol. xv. (1859), p. 353 ; vol. xvi. (1860), p. 215; vol. xvii. (1861), p. 171; vol. xvii. (1861), p. 256; vol. xviii. (1862), p. 331.
‡ These strata have been more particularly described by the Rev. J. M. Joass, Quart. Journ. Geol. Soc. vol. xxv. (1869), p. 314.
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