infiltration of silica, it may assume the character of a chert; and in like manner we may trace the production of quartzites from sandstones, and sparry amygdaloids from vesicular volcanic tufas. But it is not so easy to perceive how such mountain-masses as gneiss, mica-schist, serpentine, and clay-slate, with all their phenomena of crumpling, foliation, and cleavage, and with all their garnets, tourmalines, and other accidental minerals, could have been metamorphosed from ordinary sedimentary rocks (sandstones, shales, and limestones), even though we call in ages of pressure, heat, chemical action, and the other metamorphosing agencies which are usually appealed to. The subject of rock-metamorphism is, in truth, a most difficult problem, requiring for its solution more extended chemical research and profounder reasonings in physics than it has yet received; and in the present state of our knowledge, the ordinary geologist may rest satisfied that the slow and long-continued action of pressure, heat, chemical action, magnetism, crystallographic segregation, and similar subtle forces, are, and have been, the main agencies concerned in its production. And this much also he must ever remember, that no sooner has any rock-mass been formed, whether by chemical, vital, or mechanical means— a travertine, shell-bed, peat-moss, or stratum of sand—than it begins to undergo internal change, and this in proportion to the depth at which it may be placed, and the mineral conditions by which it may be surrounded.

Along with this internal metamorphism or change of texture, there is usually produced some external alteration or change of structure, such as "jointing," or the separation of many strata into regular blocks of shrinkage; "crumpling" and "folding" by pressure; and "cleavage," or that fissility across the bedding which characterises roofing-slate, and seems to be due partly to pressure and partly to molecular segregation. Nor must we forget to mention the tabular,

spherical, and columnar structure which characterise so many of the igneous rocks (granites, greenstones, and basalts), and which arises partly from the degree of fusion to which the mass has been subjected, but chiefly to the rapidity and conditions of its cooling. Indeed, there is no rock-mass that suffers internal change but undergoes at the same time a corresponding change in its external aspects, whether it be of chemical, vital, or mechanical origin-sedimentary or eruptive. And when once brought to the surface of the crust, it suffers still greater change of aspect through those meteoric and aqueous causes to which we have already alluded, being "weathered" by the atmosphere so as to present an aspect altogether different from its internal texture; smoothed and abraded by moving water; or polished, rounded, grooved, and furrowed by glaciers in their downward passage from the mountain-heights on which they are engendered. These external changes are generally, however, of much easier comprehension and solution than the internal; but though simpler of explanation, they are not the less important to that never-ceasing circulation of matter which it is the main duty of the geologist to trace through all its varied forms and countless modifications.

Such is a hasty glance at the formation of the rock-masses that constitute the crust of the earth, and the metamorphoses to which, for the most part, they are subsequently subjected. Amid all the changes which the rocky crust has undergone, and is still ceaselessly undergoing, there is no evidence that anything material (or that through which force manifests and exerts itself) has ever been added or abstracted from the globe. It is the same matter ever changing, in obedience to operating forces, its form and place-now solved and separated by air and water, now reconstructed by fire, now chemically severed by antagonisms, now united by affinities,

now built up into myriad forms of beauty and wonder by life, and now scattered again by dissolution and decay. Comprehending this incessant circulation, and the means by which it is brought about, the student of geology has the key to the solution, not only of the mere formation and metamorphism of rock-masses, but to the whole history of cosmical change which it is the province of his science to interpret. And where, from deficiency of knowledge or want of mental power, he is unable to reveal the whole of that marvellous history, it is something at least to be able to trace in dim perspective what is altogether shrouded in darkness to the uneducated and non-scientific mind.

BY THE SEA-SHORE.

FOR the thoughtful and inquiring the sea-shore has endless attractions; to the student of nature a thousand inducements to frequent its scenes. The stilly loneness of the long level sands on a summer gloaming, the pulse-like ebb and flow of the tide, the hazy outline of the far-distant horizon, or the shimmer of the moonlight on the midnight waters, offer an ever-present theme for thought and a boundless field for imagination. On the other hand, the myriad forms, half vegetable, half animal, that clothe the rocks, the strange and varied life that throngs the pools and reaches, the sea-fowl that soar round the cliffs or dose dreamily on the islets, and the creeks and caves and crags, with their ever-shifting lights and rugged outlines, all afford inexhaustible themes of study to the lover of nature. But it is to the geologist, more perhaps than to any other, that the sea-shore displays its irresistible allurements, and affords its most valuable lessons. In the quarry and the railway cutting, or in the ravine, he may trace the sacred letters of that great stone book which he seeks to decipher, but on a nar. row and restricted scale compared with the sea-shore, where miles of unbroken section are fully exposed to his view. Not only in the cliffs above can he study the succession of deposits, and the mode in which they have been rent and

bent and broken by vulcanic forces, but at tide-ebb, on the shore beneath, he can see, as on a ground-plan, the manner in which these rock-masses are disposed. And not only does the sea-shore afford this clearer and fuller insight into the doings of the past, but in its waste and degradation in one district, and in its accumulations and deposits in another, we get intelligible evidence of the means and processes by which geological change is effected, and new distributions of land and water secured. Let us glance for a while at the shores of our own island in corroboration of these views, and as an inducement to the young geologist to betake himself betimes to the sea-side.

And while geology is his prime object, he will find along the shore and the seaboard-that narrow belt of amphibious bordering land-a thousand things at once to gratify and to instruct. The maritime and marine flora and fauna, the picturesque creeks and coves and cliffs, the old grey ruins of keep and castle, the quaint and curious features of the little fishing village, and the busy bustle of the thriving seaport, meet him at every turn, giving new themes for thought, and a zest to his more immediate labours. "Round the shores of Great Britain" was an early ambition of ours, and full fifteen hundred miles during summer holidays we had accomplished, when the day of paralysed limbs came, putting an end to this as to many other long-cherished intentions. But though closed to one it is open to others; and where we cannot again be, we may at least be able to induce others to go for the same purpose, and with equal gratification. Naturalist, archæologist, ethnologist, poet, and painter, will alike find it a fertile field for their research : to the geologist who would gain a clear conception of the structure of the rocky crust its study is indispensable. Along this little island of ours there are illustrations of every system and formation; sections which no artist could