Transactions of the Geological Society, 1st series, vol. 3/Plates and Maps
Map of Sky.
Fig. 1. Shows the marble limestone of Kilbride incumbent on the syenite, and succeeded by alternations of shell limestone and schist.
Fig. 2. Shows the alternation of marble limestone and shell limestone at Borrereg.
Fig. 3. Shows beds of Lydian stone covered by trap: at Duntulm.
Fig. 1. Illustrates the variation of the compass on the summit of Glamich.
Fig. 2 & 3. Show how a mass of trap overspreading a series of flat strata might by the casual wearing away of parts, give the appearance of having been deposited at different eras.
Fig. 1. Shows the manner in which the sandstone of Strathaird is stratified.
Fig. 2. A compound trap vein at Strathaird.
Fig. 3. Horizontal strata of sandstone intersected by trap veins: at Strathaird.
Fig. 4. A diagram shewing the supposed geological structure of Sky.
Crystals of Uranite from Cornwall.
Geological map of the N. E. of Ireland.
Shows the geological connexion between the W. of Scotland and the N. E. of Ireland.
Sectional views along the N. E. coast of Ireland.
Sectional views of Kenbaan head and bay, on a larger scale than in the former plate.
Ground plan and section of Birch-hill colliery, near Walsall, Staffordshire.
Map of Glen Tilt.
The Map exhibits the general appearance of the rocks which are visible at the surface. These are distinguished by colours, of which an explanation is given in the margin. I may remark that the same colours are used to represent the same substances throughout the whole of the plates belonging to this paper.
The Map does not pretend to give every rock which comes to the surface, since the spaces which many of them occupy are so small as to have rendered such a detail impracticable. I have omitted particularly many of the small masses of quartz rock which are visible on the granite, as they would, instead of elucidating, have obscured the explanation which this map is designed to give. The southern side being of a more simple construction admitted of a more real detail, yet in this also I have not pretended to lay down the perpetual and often minute alterations of the schistose rocks with the limestone, since there would not have been room for this purpose: I have contented myself with indicating them in a general way.
In the method used in colouring, I have defined each colour in those places where the rocks themselves are visibly defined. Where these boundaries are uncertain from the covering of soil or other causes, the colours are undefined. The uncoloured parts which lie near the river are intended to represent the alluvial matter, although there is little doubt that the junction of the limestone and the granite exists below it. I have detailed as well as I could the several points where that junction is actually visible: greater accuracy would have been impracticable on the map which I was obliged to make use of, with which my own measurements were often at variance. But it is a matter of no moment for the purposes of this paper, since its object will be equally accomplished whether there are twelve or thirteen junctions visible, or whether Forest lodge is three or four miles from Gow's bridge. I have only marked one or two masses of porphyry, as a knowledge of their places was of no moment.
This and the six following plates are intended to represent some of the most remarkable circumstances which attend the junctions of the granite with the stratified rocks. They consist of the most interesting portions, and of those which appeared to be the best calculated for explaining the different appearances which are to be seen at these points. With regard to the method used in sketching them, I must add that they are only eye views. Since the greater number of them are represented as if drawn from a point at right angles to the horizon, when they were necessarily taken at an angle often far less than this, it is plain that they will transgress the laws of perspective. But this will produce no alteration in the view they give of the geological facts, however it may derange their graphic accuracy. I have detailed the portions, most frequently, as if they had been detached, although they form in fact parts of continuous rocks. It is plain, when the magnitude of the objects represented, often extending to 40 or 50 feet, together with the minuteness of the fractures which they exhibit, often descending to the tenth of an inch, is considered, that drawings on so small a scale could not be expected to give an accurate detail of all the points in such a space. The leading features however have been marked with as much accuracy as the nature of the subject admitted, and whatever omissions or alterations may have been made, no liberties have been taken which could in any respect misrepresent the facts described in the paper and visible in the places noted.
At the upper end of this figure the granite appears to alternate with the schist. Tracing it further the true nature of the mixture is evident. The portion was selected to show a fact which has been mistaken for an alternation of schist with granite.
Represents a disturbance produced in the usual continuity of the schist and limestone, the schistose beds being abruptly broken off at their lower end. It also shows the detached points and lines of granite which are described in the paper, the limestone at the same time bearing indications of its original laminated structure, although the bed is not only here in a vertical position, but its course is also at right angles to the ordinary course of the beds which constitute the southern side of Glen Tilt.
Represents the splitting of the limestone bed into three parts, with the intrusion of two masses of granite. A confusion of the granite, schist and limestone is also visible on one side. It further represents the flexure of the limestone and the red lines of granitic matter running parallel to it, of which detached specimens are in the museum of the Society.
Represents a more perfect example of the red lines which are found in the limestone, and these serve at the same time as indications of the bending which the limestone has undergone. Minute granite veins resembling in their composition these red parallel lines are found traversing the fragment of schist, which has lost its usual conformable parallelism to the calcareous bed.
Is selected as a representation of that utter confusion among the substances contained at the junction which almost eludes the powers of the pencil. It contains examples of all the variations which in the former drawings have been separated from each other, and in addition to those is shown an instance of the compression of the limestone bed.
To preserve uniformity in the colours which represent the different rocks I have here also tinged the limestone blue. It is white in nature. It serves to show the entire loss of the stratified character which the limestone so often undergoes in the vicinity of the granite. Specimens from this junction are also in the Society's collection.
The sections in this and the next plate are intended to represent the relative positions of the rocks, and they are founded on numerous observations throughout the Glen. The alternations are not laid down as real, nor is there any pretence to conjecture dimensions which could not be measured.
Fig. 1. Represents the case occurring at Gow's bridge where the limestone and schist are found on each side of the river. It may be said that the water has not yet wrought its way to the junction.
Fig. 2. Represents the case which predominates throughout Glen Tilt. The river here divides the stratified rocks from the granite, and has exposed the various junctions which are laid down in the map and described in the paper. It is easy to see how these will sometimes consist of limestone, sometimes of schist or of quartz rock, and sometimes of all the three substances, with the granite.
Fig. 3. Represents the case which occurs at Cairn a'chlachan, as well as in numerous other situations, and it explains the otherwise puzzling phenomenon of the apparent alterations between the quartz rock and the granite. It is easy to see that the schistose strata, once possibly lying much higher on the granite, have remained in some places while they have disappeared in others.
Fig. 4. Is intended to represent the probable confusion which exists at the junction of the stratified with the unstratified rock. The drawings of the actual appearances at the junctions which are visible, will show that there is abundant ground for such an imaginary section.
Fig. 1. Represents the actual superposition of the rocks at the Criny, serving to justify the ideal sections which have been already given. The rocks being inaccessible are not however measured. Other indications of a similar order occur in different places, but I thought it unnecessary to represent them.
Fig. 2. Is intended to represent the probable appearance of the strata had they been deposited on the granite. Since the same beds of quartz rock, schist and limestone, are parallel among each other, they should if they had been thus deposited been also found parallel to the granite. But the actual appearance is shown in
Fig. 3, Where the strata, sometimes of limestone, sometimes of quartz rock, sometimes of schist, are found in contact with granite.
Represents the extraordinary fact occurring near Gow's bridge; the interference of beds of hornblende, schist and marble.
Geological map of the South-Western Part of Somersetshire.
Fig. 1 & 2. Represent the seeming alternation of red marle and lyas on the coast of Somersetshire.
Fig. 3 & 4. A map and section of part of Lincolnshire.
Lord W. Seymour's Clinometer.