The result of these operations have been to exhibit still further the highly fossiliferous character of the Nitberry Silurians, and to give ample indication of a very varied and curious crustacean fauna altogether new to Palæontology. Molluscous remains of well-known Upper Silurian genera have also been obtained in sufficient numbers to prove the affinities of the beds; and indications of both an aquatic and terrestrial flora seem by no means rare throughout the strata.

Another fact fully established by the exploration is, that while the lower beds exhibit the closest paleontological relations with the Ludlow beds of England, the upper pass insensibly-and without any marked boundary, lithological or paleontological-into flaggy tilestones which are the undoubted equivalents of the lowermost Old Red of Forfarshire.

The specimens obtained during the explorations have a threefold value:1st, as proving the true Upper Silurian epoch of the Nitberry strata, and thus affording a clue to the investigation of other Sub-Devonian tracts in Scotland which are yet but imperfectly understood; 2nd, as adding new forms to the life of a former epoch, and thus extending the boundaries of our zoological knowledge; and 3rdly, as enabling the Government palæontologists, who have recently published their first monograph on the Eurypteridæ, to understand more clearly the nature of this curious family of Crustaceans, and to correct what must now evidently appear as misinterpretations of their structure and affinities.

Arranging in order the fossils obtained by Mr. Slimon, we have of
PLANT REMAINS :-

Undetermined organisms, apparently Crustacean or Amorphozoan.

In none of the beds examined, nor during the whole of Mr. Slimon's pre

vious explorations, which have extended over several years, has there ever been detected any trace of an ichthyolite-fish-scale, fin-spine, or tooth-a noticeable fact, considering the highly fossiliferous character of the strata, and the indications that many of them give of littoral as well as of deep-sea conditions of deposit.

Looking at the palæontological value of Mr. Slimon's discoveries, and the additional interest they have conferred on Palæozoic Geology, your Committee would respectfully urge upon him a continuance of his labours, conjoined with the hope that, if at all compatible with the other requirements of the Association, a further grant of say £10 or £20 should be made to assist in so desirable an object.

Report on the Results obtained by the Mechanico-Chemical Examination of Rocks and Minerals. By ALPHONSE GAGES, M.R.I.A., Curator of the Museum of Irish Industry.

I HAD the honour of bringing before the Section at the last meeting of the Association at Leeds, a short paper entitled "On a Method of observation applied to the study of Metamorphic Rocks, and on some Molecular Changes exhibited by the action of Acids upon them." The principal feature of this method of examination consisted in exposing thin plates of rocks, or crystals cut in certain directions, to the slow action of solutions of acids or alkalies of different degrees of concentration, under such varied circumstances as the special characters of each rock may suggest. The general result of this action was the gradual removal of some or of all the bases, a residue being left, the structure and composition of which indicated the mode of formation of the original rock.

The idea of submitting rocks or minerals to the action of various solvents is not new. But hitherto experimenters have operated upon the powdered mineral. I operate upon fragments which exhibit not only the chemical constitution of the substance under examination, but what is in many instances of still greater importance, the mechanical constitution also. An example will explain still better the difference.

If we powder a piece of alum and put it into water, it will dissolve, and so far as the appearance presented by the powdered mass, uniformly. But if we take the same piece of alum, and instead of breaking it up, grind a flat surface upon it, and place it, as Daniel did, in water with its polished face downwards, the water will act upon that face very unequally; after a time crystals will stand out in relief, and what looked like a homogeneous crystalline mass, will be shown to be made up of a confused mass of interlaced crystals cemented together.

Observers have no doubt dissolved minerals in fragments as well as in powder, but they have not, so far as I am aware, done so with the object of studying the peculiar mechanical arrangement of the components of rocks, and certainly have not done so as a methodical system of examination.

If, as Daniel's experiments show, we may learn much regarding the molecular structure of even a crystalline mass of a homogeneous substance by the manner of dissolving it, how much more so must this be the case with such complex mechanical mixtures as most rocks are! Before detailing the experiments which I have made during the past year, I may observe, that although the application of this method of examination (which, for want of a better word, we may call mechanico-chemical) is limited to a certain number 1859.

F

of rocks, it may be advantageously employed as a kind of preliminary qualitative analysis, in the case of the majority of sedimentary rocks, whether metamorphic or otherwise, before reducing them to powder, in order to analyse them by the ordinary method. The slow and prolonged action of acids on minerals or rocks composed of a mixture of minerals, or even those mainly composed apparently of one mineral, enables us sometimes to discover substances which would otherwise have passed unnoticed, and the constituents of which would consequently be confounded with those of the rest of the rock.

The number of rocks which resist without decomposition the prolonged action of acids such as HCl and HO SO, at various temperatures, up to their boiling-point, is extremely limited; and this is especially the case if fragments of rocks be subjected to the alternate action of the two acids. Those especially which have undergone a slight alteration, such as the commencement of the formation of hydrated minerals, always yield to such treatment.

A great number of rocks, consisting of aluminous silicates or silicates of lime or magnesia, frequently leave, after treatment with acids, skeletons which show us the manner in which many minerals may have been decomposed, the residues which they left often serving as the basis of new formations. In examining calcareous rocks containing such skeletons, it is necessary to use dilute acid solutions, sometimes indeed extremely so; as concentrated acids might in many instances decompose the skeletons, especially if they appeared to contain hydrated silicates.

The rocks which I have submitted to examination since the last meeting of the Association may be classified as follows:

1. Comparative examination of the residues of Permian magnesian limestones from ten localities.

2. Comparative examination of the magnesian limestone of Howth, Co. of Dublin, contrasted with those of the Permian localities.

3. Magnesian limestone conglomerate from Downhill, Co. Londonderry. 4. Examination and analysis of a pseudo-dolomite, found at the junction of the trap and carboniferous limestone, at Stone Park Quarry, 2 miles north of Six Mile Bridge, Co. of Limerick.

5. Experiments on the composition and structure of the residues obtained from the Calp or middle limestone, Co. of Dublin, and of the lower limestone shales of Drogheda.

6. On chloritic slate, and metamorphic limestone derived from it.

7. On a metamorphic limestone containing garnets reposing on the granite near Gweedore River, Co. Donegal.

1. Magnesian Limestones from Permian Localities.-There appears to me to be considerable confusion in the minds of some geologists regarding what are called Magnesian Limestones. The terms Dolomite and Magnesian Limestone, in the sense in which they are sometimes employed, seem to imply a similarity in the mode of formation. This is, however, far from being the case. Most limestone rocks, whatever may be their origin, contain some magnesia; and even recent corals and marine shells have been found by the investigations of Dana and Forchammer to contain some.

I have no intention to propose a nomenclature of magnesian limestones; I merely wish to trace the distinctive characters of some of those rocks by means of the residues which they leave when treated with acids, and which are often the only witnesses which could instruct us as to the mode of their formation. Some of these residues are very characteristic; thus the Permian are ochrey, and always contain fragments of hyaline quartz, sometimes rounded on the angles. Those, on the contrary, derived from magnesian limestones formed either by infiltration or in a tranquil medium, and

Tabulated Statement of the Characteristics of the Permian Magnesian Limestones examined, and the Proportions of Residues which they contain.

10. Exhall,

9.30 Residue of a highly plastic ochrey clay of a yellowish buff-colour, and containing a fine debris of transparent quartz.

28.35 Residue consisting of a ferruginous clay of a violetred colour, intermingled with about its own weight of debris of quartz.

8.70 Residue composed chiefly of very small fragments of transparent quartz, with some opalescent ones also, as large as a pea. Traces of yellow ochre.

1.28 Yellowish-brown clay and minute fragments of transparent quartz.

1.66 Light-brown

ochre, with

some fragments of hyaline quartz.

1.30 Very minute granular fragments of quartz, with a light brown-coloured ochre.

1.20 Very minute grains of quartz, with light-brown ochre,

5.55 Light-brown ochre, with small angular fragments of hyaline quartz.

22.00 Red ochre, with some fine hyaline quartz sand.

Co-Sandstone formed of fine 21.53 78-47 If we reverse the numbers

quartz sand cemented

ventry.

by carbonate of lime

and magnesia.

representing the sand and carbonates, we shall have a magnesian limestone of the

same character as No. 9.

under the influence of the decomposition of other rocks, contain, in the majority of cases, crystalline substances in a whole state, or partially decomposed silicates.

Having just indicated the comparative distinctive characters of the residues left by the magnesian limestones of different kinds, I will now proceed to describe those of the Permian in detail.

The magnesian limestones of the Permian group which I have had an opportunity of examining, leave, when treated with hydrochloric acid, more or less abundant residues, offering the same lithological characters. These residues are ferruginous clay, varying in colour from deep red to very pale yellow. These variations of colour are due to the relative proportions of sesquioxide of iron present, and sometimes to that of manganese also. The residues contained besides fragments of transparent quartz, which may be separated by washing. The oolitic characters which some of those magnesian limestones assume are always due to those fragments of quartz, which serve as nuclei around which the deposit of carbonates is formed. The quantity of residue sometimes exceeds 30 per cent., and often does not amount to per cent.; but whatever may be the quantity of the residue, its lithological characters remain always the same.

The following Table contains the results of my examination of each of the specimens of magnesian limestone from Permian localities.

No. 3 in the Table illustrates very strikingly the origin of the oolitic structure in calcareous rocks. When a fragment was exposed for a short time to the action of hydrochloric acid, so as to remove part of the lime, the grains of sand were observed standing in a kind of hollow shell. It differs, however, from the generality of oolitic rocks, in which the grain of sand or matter forming the nucleus is surrounded by concentric layers of calcareous matter. In the rock under notice, the grains of sand appear, so far as can be judged by means of a lens, to have been simply imbedded in the cementing parts.

2. Howth Dolomite.-The dolomite of Howth, Co. of Dublin, belongs to the carboniferous series, and rests on Cambrian slates. It is of a light yellowish-brown colour and has a compact crystalline texture, with many cavities, however, which are filled with well-developed crystals of bitter-spar. When examined with a lens, it appeared to be formed of a series of irregular serrated layers, sometimes containing oxide of manganese in more or less quantity. On being treated with acetic acid, it divided itself into small granular crystals of bitter-spar, resembling an extremely fine sand. It thus presented all the characteristics of true dolomite.

Treated with dilute hydrochloric acid, it left a residue never exceeding 3 per cent., and consisting of a reddish-brown ochrey clay mingled with crystals of quartz, which were separated by agitating the residue in water. They consisted of very fine acicular crystals of opake quartz, having a fibrous arrangement, the edges of some of the crystals being somewhat eaten away. Washed several times with hydrochloric acid, and then treated with hydrofluoric acid, these crystals yielded an appreciable quantity of alumina, oxide of iron, lime, and magnesia, a circumstance which suggests that they may be the relics of some augitic or hornblendic rock. The Rev. Prof. Haughton, to whom I submitted these crystals, and who examined them, considered them to be "fibrous quartz, and such as occurs in the minute veins of quartz in the slate rocks of which the Hill of Howth is formed." What is most remarkable in connexion with those crystals, is the constancy with which the residue is found disseminated throughout the whole dolomitic mass of Howth.