It has always appeared to me, that the study of the stratification of rocks and of mountain masses, ought to be one of the principal objects of a geological observer. Many of the most important facts in geology have been ascertained by the consideration of the position of strata. Among these facts, the relation existing between the direction and the inclination of the strata and the unstratified rocks, to whose presence the change in the position of the beds from an horizontal to an inclined, and sometimes even to a vertical situation, is now generally attributed, is one of the most conspicuous. It is only by an accurate determination of the position of the strata in any mountain-chain that the real direction of the line of elevation of that chain or its mineralogical axis may be determined.

During the researches which I have made upon this subject since the composition of my former paper, printed in the first Part of this Volume, so many and so varied inquiries arising out of it, and essentially connected with it, have presented themselves, that I have been compelled to make a total change in the plan I had then laid down for the completion of the present section of my communication. I found that many subjects to which I there alluded might, with propriety, be omitted in the present case, as constituting little more, in reference to principles, than illustrations, however interesting they might be when viewed as properties of geometrical figures. On this ground, therefore, I have cancelled a considerable number of properties of the Spherical Conic Sections, and retained only one or two for the purpose of illustrating the method of discussing the properties of those curves. The remarks I intended to make upon the singular points of spherical curves, the geometrical signification of certain symbols, and other inquiries collateral to these, have grown into systems of themselves, or been attached to other dissertations (either wholly or partially completed), to which they seemed to be as closely allied as even to the present subject.

1. On the 17th January 1831, Mr Arthur Trevelyan communicated to the Royal Society of Edinburgh a paper, entitled “Notice regarding some phenomena observed during the Cooling of certain Metals placed in contact with Lead.” This was the first account published of the remarkable discovery made by that gentleman, of a most curious class of phenomena, which till then was unknown to the scientific world. This paper was afterwards published, with some additions, in the 12th volume of the Transactions of that body, under the title of “Notice regarding some Observations on the Vibrations of Heated Metals.”

2. Mr Trevelyan had, in February 1829, first observed the phenomena just alluded to, which consist in certain tremulous motions accompanied by sounds, often highly musical, excited in many metals while hot, placed in contact with lead or tin, at a lower temperature.

The inquiries of scientific persons into the various departments of knowledge, have very generally for their sole object the investigation of truth, and the progress of science, without any reference to trade or the usual business of life. Hence to the practical man they seem generally dry and without interest, his mind being directed towards immediate utility, public or individual. Should I in the course of the present inquiry, (which in its essential nature is strictly anatomical and physiological, and can be investigated with advantage only by the anatomist and physiologist,) seem to the strictly scientific to have paid too much attention to practical details, apparently unconnected with the scientific part, and consequently out of the line of my vocation, my apology is, that in the course of the observations, I found this to be unavoidable, although my utmost efforts were made to prevent the inquiry running into this laborious and expensive train; and I entertain a firm belief, that, on a careful perusal of the observations, taking all matters into consideration, it will be found that I could not have done otherwise.

In a paper on the Monochromatic Lamp, &c., read before this Society on the 15th April 1822, and published in their Transactions, I recorded some of my earliest experiments on the action of coloured media on the Solar Spectrum. These experiments were continued at irregular intervals, with the view of obtaining distinguishing characters of coloured media, of investigating the cause of the colours of natural bodies, and of examining more correctly the phenomena of the overlapping colours of equal refrangibility, the discovery of which I had announced in the paper already referred to. The results to which I was conducted on the two last of these subjects, have been already communicated to the Society in two papers, one on the Analysis of Solar light, and the other on the Colours of Natural Bodies.

Having frequently had the pleasure of examining the very interesting and highly valuable collection of volcanic specimens brought from Iceland, and presented to the Royal Society by Sir George Mackenzie, I was gratified in an opportunity which was afforded me, at the close of 1830, of visiting the Lipari Islands, the only other great repository of obsidian in Europe, and of there collecting the suite of specimens which I have now the honour of presenting to the society. Unfortunately, it is far from complete as a geological series; for the difficulties occasioned by the tempestuous state of the weather, and the impossibility of getting to the more remote islands in a small boat during the month of December, limited my movements extremely, and forced me to confine my observations to the two principal islands of the group, viz. Lipari and Vulcano. I do not therefore pretend to give any geological account of these islands, or even of the two on which I landed. My object in collecting, and consequently also in presenting to the Society, this series of specimens, was to afford the means of comparing with each other the productions of the two great obsidian districts of Europe; those from Lipari, which are now on their table, with the Iceland suite already in their possession.

There are few of the applications of optical science so universally interesting as that which has for its object the explanation of the colours of natural bodies. Sir Isaac Newton was the first person who ventured to refer to one general principle all the variety of colours which are found in nature; and he has maintained his opinions on this subject with a confidence in their accuracy which seems to have confounded his adversaries: For while his analysis of light, the most perfect of all his labours, exposed him to the most harassing controversies, his theory of natural colours, the least perfect of his speculations, was allowed to pass without examination or censure.