tinguished from ruby, minerals of the cubic system being optically similar in all directions, and consequently exhibiting no pleochroism, while ruby gives pink-red and yellow-pink axis-colours. Glass imitations of emerald, ruby, or sapphire may be similarly detected, and this too in cases where the nature of the specimen renders other tests undesirable.

We may note in conclusion that Haidinger's first experiments were conducted with a plain cleavage-prism of calcite, at one end of which a stop with a square opening was placed. The dichroscope can now be obtained from London mineral-dealers or opticians, at prices varying from about 15 to 20 shillings.

PART III.

THE EXAMINATION OF ROCKS.

"As for the earth, underneath it is turned up as it were by fire. The stones thereof are the place of sapphires, and it hath dust of gold."-The Book of Job.

No arrangement can pretend to define and separate those objects which the hand of nature has neither defined nor separated."-JOHN MACCULLOCH, A Geological Classification of Rocks, 1821.

CHAPTER X.

INTRODUCTORY.

WHILE a mineral may be to a large extent discussed and determined in the laboratory, a rock, considered as a part of the earth's crust, and not as a mere aggregate of chemical compounds, requires a very full knowledge of its mode of occurrence before it can be properly treated of and described. In fact, after a study of a number of type-specimens, the student is recommended to go out to some well-described district, and to endeavour to recognise the varieties of igneous and sedimentary rocks by careful observation in the field. In this way alone can he appreciate the various modes of weathering, the massive or minuter structures due to jointing, the smooth or rugged outlines, that characterise the masses of which his hand-specimens form a part. He will meet with many difficulties of determination, and will procure a store of well-selected material on which to work during less propitious days. Questions will arise, even during microscopic examination, that will send him back to gain further information in the field; and in the end his investigations will have far more geological value to him than any knowledge gathered in type-collections or museums.

The notes that follow presuppose that the specimens have been collected in the field; that at any rate something is known about

their mode of occurrence and their relation to other parts of the same mass. Weathered specimens should be avoided as a rule, but often reveal structures hidden in the unaltered portions. Collections made from stream-beds or taluses are often useful for showing the general character of a district; but rocks so gathered are seldom of value for detailed study. Nothing short of striking the rock-mass in situ with the hammer, and taking in with the eye its position and surroundings, even to the broader features of the landscape, should content the geologist who would follow worthily the founders and masters of the science.

The points of interest presented by various types of rock will be dealt with later. Broadly speaking, in the case of sedimentary rocks specimens should be collected showing weathered surfaces and also freshly exposed bedding-planes, since minute structures, fossils, &c., are best made out by a comparison of these. In the case of igneous rocks, specimens must be taken from the centre and from the edges of any dyke or lava-stream, and contactphenomena repay the closest examination.

The pocket-lens and the knife are, in the field, of paramount value. To one, moreover, familiar with microscopic sections, a number of structures and mineral-forms will reveal themselves with unexpected clearness when a fairly smooth surface of the rock is examined with the lens alone. The pocket-knife must be used freely, as in the case of minerals, in estimating the hardness of a rock. The angle of a steel hammer, drawn across the face, often gives similar information. All rocks tend, however, to have a hardness a little below that of their principal constituents, owing to looseness of texture or development of decompositionfilms between the grains. But granular limestones can at once be distinguished by the knife from the unscratchable quartzites; basalt, which is scratched with some difficulty when fresh, can in this state never be confused with black limestone or compact dark shale-mistakes that have been often made during the hurried examination of hand-specimens. The acid-bottle, owing to risk in carrying and the necessity of employing heat in many tests, is seldom of great advantage in the field, though in the laboratory it should always be at hand. Dry citric acid, as already described, may, however, be conveniently carried on excursions.

CHAPTER XI.

ROCK-STRUCTURES EASILY DISTINGUISHED.

THE structure of a rock naturally demands the first attention. In the majority of cases evidence can be obtained in the field respecting the mode of origin of the specimens collected, and only their systematic position will remain to be determined. Rocks may be divided according to their more obvious structural characters into the following groups :

GROUP A.-COARSELY FRAGMENTAL ROCKS.

Stratification may or may not be present. In gravels and con glomerates current-bedding should be looked for. The position of the pebbles, with their longer axes lying in planes parallel to one another, will often give a clue to the bedding or the local dip. The degree of rounding of the pebbles, their average size, and any evidence of crushing and recementing that they may have undergone since their deposition, are structural points that may prove of considerable value.

If the fragments forming the rock are very angular, we may be dealing with an old talus unworn by water, with a volcanic agglomerate, or with a mass brecciated subsequently to its consolidation. The nature of the fragments will probably decide the first and second cases; in the third, we must search for slickensided surfaces, which should be abundant, and must study the relations of the fragments one to another. If the rock is a friction-breccia, detached portions of larger blocks will be found lying suggestively near their parent masses, the interspaces being filled with a sort of fault-rock. Other blocks will be faulted without being actually broken asunder; but there are, of course, cases, especially of fragmental rocks that have been affected by earthmovements, which are particularly hard to settle. Near faults, in contorted mountain-districts, and especially where softer rocks abut against harder and more resisting masses, this brecciated structure may be looked for and expected.

GROUP B.-ORDINARY STRATIFIED ROCKS.

The points to be ordinarily examined in these are the character of the bedding, which may be so fine as to produce a laminated

structure; and the degree of coarseness of the constituents. The texture of the rock may thus be granular and grit-like, or quite microgranular and compact. Fine-grained volcanic tuffs and ashes must often be classed with ordinary sediments until the aid of the microscope can be called in. Abnormally large fragments in a rock of fine texture, particularly if they consist of exceptional materials, may often be ascribed to the action of floating ice. Examples of this kind are the granite and other boulders that have been found in chalk.

Oolitic and pisolitic structures, the latter, as commonly understood, being a coarse development of the former, are comparatively common among limestones. With a lens, the concentric coats of the egg-like granules can be clearly seen, and often a central nucleus, some fossil or mineral fragment, can be detected in those that have been neatly fractured. On the weathered surfaces of some oolites, such as the Hirnant limestone, the structure is brought out with great distinctness.

On the surface of the stratified layers, ripple-marks, casts of sun-cracks and worm-burrows, footprints, and other signs of a littoral origin, may often be detected in the field. The relations of the fossils to the bedding-planes must also be observed; in some instances mollusca are found embedded in the position in which they lived in muddy banks; in some beds again, bivalves will be found commonly entire ; in others the separation of the valves and the rolling of the fossils generally will give evidence of troubled waters at the time of deposition. Derived fossils must not be overlooked.

The nature of the cementing material in a granular rock must be examined. Interesting and exceptional substances, such as barytes, have been occasionally found to play this part.

Concretions must be studied from a mineral stand-point. On being broken open, they often show shrinkage-cracks filled with products of infiltration, giving rise to a septarian structure. Fossils and various original structures are often preserved in the concretions when lost by alteration in the surrounding rock.

Lastly, the joints must be inspected. They may be filled with secondary minerals, and their bounding surfaces may at times be slickensided by earth-movements.

We must bear in mind that, associated with ordinary fragmental stratified rocks, there often occur bands of crystalline material, such as rock-salt and alabaster, which have been deposited from solution and which exhibit a massive structure.