each class in its own special element only, and which therefore die in Inyriads where the sweet and the salt waters mingle. It is almost incredible, how densely the water is sometimes peopled by these creatures, how rapidly they multiply, in what countless numbers they exist. Their skeletons and envelopes, consisting of calcareous and siliceous matter extracted from the water, are almost imperishable. They commix with the mud of the river; and, with it, they come to form the deposits of slime that fill up the channels, raise the growing islands, or add to the belt of most fertile land which increases seaward where the waters are still. As the tide advances up its channel, the waters of the river spread and flow over the surface; so that, far up the stream, where the upper waters are still sweet, the salt or brackish under-current carries the living things which float in it to certain death, and leaves their bodies behind it to add to the accumulating mud. The extensive mutual surfaces of river and sea-water which in this way are made to meet, ensure a more rapid destruction of infusorial life than could in almost any other way be brought about.

"Experiment has shown that, as far up as the tide reaches, the so-called alluvial deposit in and along the channel of the river abounds with the remains of these marine animalcules; while above the reach of the tide none of them are to be found. In the Elbe they are seen as far as eighty miles above its mouth. About Cuxhaven and Gluckstadt, which are nearly forty miles from the open sea, their siliceous and calcareous skeletons form from to of the mass of the fresh mud, exclusive of the sand; while, farther up the river, they amount to about of this quantity. In the Rhine, the Scheldt, the Mersey, the Liffey, the Thames, the Forth, the Humber, and the Wash, the same form of deposit goes on: so that, in the mouths of all tidal rivers, there are to be superadded to the mechanical debris brought down by the upper waters, the more rich and fertilizing animal spoils which the sea thus wonderfully incorporates into the growing deltas and the banks of rising mud. And thus it is seen that river-islands encroach upon the ocean, not merely in proportion to the solid matters held in suspension by the descending water, but in proportion also to the richness of the sea in microscopic forms of life, and to the volume of fresh water which the river can bring to mingle with it."

[G g.]

Referred to at page 111.

ON THE PEBBLES.

In the constitution of these common objects, the despised pebbles of our plains and the shingle of our coasts, there is much that

deserves attention. They consist very extensively of the fragments of the older rocks, chiefly the igneous, called primary; and these often include distinct masses of very pure silex, both amorphous and crystallized, from which the detached and rolled fragments become pebbles of jaspar and chalcedony. In many of these, and in the Flint nodules which characterize the Upper Chalk, there are recondite wonders, not only of deep intrinsic interest, but carrying us back to the view of a degree of heat affecting the ancient surface of our globe, so high as to be immensely beyond the capacity of the vegetables and animals of the present creation ever to have sustained. That heat must have been such as, combined with the pressure of an atmosphere, probably of carbonic acid gas, and that made heavier by enormous condensation, enabled the waters to hold in solution a quantity of pure siliceous earth, so great as to have furnished the masses of pebbles (chert, flint, jasper. and agate,) which so abound in many countries. It has long been observed that many varieties of chalcedony (called moss-agates, mocha-stones, and perhaps by other names,) show substances embedded and beautifully spread out, as if suspended in a fluid, or like the finer sea-weeds (algae and conferva) as delicately displayed in a lady's album or hortus siccus. These, after making all reasonable deduction for mineral infiltrations, are indubitably organized bodies; and which, with some exceptions from the insect tribes, were referred to the Confervæ, or other cryptogamic plants. See a Memoir, with beautiful coloured plates, by the late Dr. Mac Culloch, in the "Geol. Transact." first series, vol. ii.

But the recent investigations of a distinguished Geologist and Microscopist, Mr. James Scott Bowerbank, have thrown a satisfactory light upon the constitution of the many species and varieties of the well-known substance sponge, have established its claim to belong to the animal kingdom, and have detected numerous instances of spongeous bodies, in the fossil state, and always embedded in flints, agates, or other siliceous stones. Now sponges (and in some degree other Porifera, as the Alcyoniums,) have in their internal structure minute aggregations of silica, generally in the shape of needles or darts, and therefore called spicula. There is a manner of action, we presume to call it a law, which may be named homogeneous attraction; the result of which, in the waters of a former state of the globe, was that these phytozoic bodies attracted around their own substance the silica dissolved in the hot waters, bringing it more or less gradually, and often it would appear most rapidly, into the solid state in these masses they themselves being enclosed. This subject has been illustrated by Mr. Bowerbank, in various Memoirs, published in the Geological and Microscopical Societies' "Transactions and Proceedings," the "Annals and Magazine of Natural History,' and the "Microscopical Journal;" in the years 1841 and 1842, and subsequently.

The inference to Geological theory from these facts is a strengthening of the general argument for believing in a high antiquity of the earth; from its having existed and been the abode of organized creatures, whilst it was under conditions incompatible with the life of its present inhabitants.

A similar fact we have which, though not bearing in the same way on the estimate of past time, is highly interesting as a proof of the abundant solution of silica, in the heated waters of a former terrestrial condition. This is the Marquis of Northampton's discovery of microscopic shells of the nautilaceous family, in the heart of flint-pebbles. See the preceding Note.

[H.]

Referred to at page 911.

ON ANCIENT GLACIERS AND THEIR EFFECTS.

Ar the time when the second edition of this book was published, Dr. Lewis Agassiz was exploring the northern mountains and valleys of our island, and applying the hypothesis which he had built upon the foundations of MM. Venetz and De Charpentier, of the presence of extensive glaciers, and their action in glazing and marking the surfaces of rocks, and in the transport and arranged deposition of pebbles, bowlders, and masses of stone retaining the edges and angles, thus showing that they had not been much if at all rolled. He detailed many of his observations and opinions to the Geological Section of the British Association, at Glasgow, in September, 1840. Similar communications he also made to the Geological Society of London. In the same year he extended his laborious researches, aided and in great part accompanied by Dr. Buckland, Mr. Lyell, Mr. Charles Maclaren, and other eminent Geologists, in Scotland, the north of England, and in some parts of Ireland; applying the principles which he had derived from the study of his native Alps. In the autumn he published at Neuchâtel his interesting work, "Etudes sur les Glaciers; with an Atlas of magnificent plates, executed with rigid accuracy as well as beauty. Most or all of our great authorities gave their approbation to his theory, to some extent; on the degree however of that extent, they varied. All viewed their frank and pleasing visitor with honour and esteem; all admired his extensive and minute knowledge, his indefatigable laboriousness, his prompt and lively talents in communication, and his ability in supporting his doctrines. Without presumption, I trust, it may be said that those doctrines and their application will stand the test of time to a considerable extent, but not so far as the distinguished man

carries them. Many rock-surfaces, polished, scored or grooved, and many arcuated aggregations of travelled stones, are well proved to be in such conditions as authorize the belief, that glaciers had formerly existed in the higher latitudes of the British Isles, producing similar results to those now observed in the Alps; and that an elevation of climatic temperature, immediately preceding the creation of the present tribes of nature, obliterated the cause and left us the effects.

A chief temptation of accomplished yet sanguine minds is the pushing too far of a good principle: and this temptation finds scarcely anywhere more scope and occasion than in relation to Geology. It can scarcely be doubted that Agassiz has rested too much upon the simple glacial action, the dry movement; and has not sufficiently introduced the evidences of the same action urged by torrents and tides. The principal Geologists of the continent, including Switzerland, have the most severely impugned this theory: and, in our country and America, its claims are a good deal limited yet with great liberality to its talented advocate.

Mr. Charles Maclaren of Edinburgh is the author of a compendious summary of Dr. Agassiz's volume, and his other communications on this subject: "The Glacial Theory of Professor Agassiz ;-being an Outline of the Facts and Arguments adduced by him to prove that a Sheet of Ice enveloped the Northern Parts of the Globe at a recent Geological Epoch." Edinb. 1841. But this small treatise is not a mere analysis or abridgment of the "Etudes;" it contains judicious observations upon the theory, and a testing application of it to geological phenomena in the neighbourhood of Edinburgh. The author observes, "These very original and ingenious speculations of Professor Agassiz must be held for the present to be under trial. They have been deduced from a limited number of facts, observed by himself and others, and skilfully generalized; but they cannot be considered as fully established till they have been brought to the test of observation in distant parts of the world, and under a great variety of circumstances. Supposing the theory to be substantially sound, the magnitude of the consequences it involves will undoubtedly bring objections to light, which may render modifications necessary, both in its principles and its details. In the meantime, it assists us in resolving some difficulties. It contributes, in a greater or less extent, to explain the dispersion of erratic blocks, the bizarre situations they occasionally occupy, the banks of clay and gravel found on the sides and at the mouths of valleys, the stric, polishing, and grooving, observed on the surface of rocks in situ, and of large stones in the till" [a Scottish term for superficial deposits of clay, sand, and gravel]; "and it promises to throw light on what is at present a very obscure subject, the origin of the older and of the newest alluvium." Page 38.

D D

Professor Hitchcock urges the necessity of taking a larger account of aqueous power, and judges it "extremely probable that all the phenomena of what has been called Diluvial Action, are the result of the joint agency of ice and water; the water resulting from the melting of the ice." "Elementary Geology," sec. ed. p. 192.

More recently, Dr. Hitchcock says "It will need important modifications-I doubt whether any one [of the American Geologists] is ready to adopt the unmodified glacier theory of Agassiz: although doubtless many will admire the ingenuity and indomitable perseverance of that distinguished naturalist, and thank him for the great light which his labours have cast upon the phenomena of drift." "Amer. Journ. of Science ;" Oct. 1842, page 398.

All must regard Mr. Murchison as peculiarly qualified to hold the balances, and mark the preponderance of argument on any geological question. In addition to his great attainments in geological knowledge, his tact in judging of facts and opinions, and his practical skill in exploring and observing, he has, in the two years 1840 and 1841, travelled over large tracts in the northern part of the Russian Empire, both European and Asiatic, employing his practised eye and patience, expressly for the advancement of this branch of natural science. In his Discourse as President of the Geological Society, delivered February 18, 1842, he has discussed the Glacial Theory at considerable length. I select a few extracts, the briefest possible in consistence with the requisite perspicuity.

"The Glacier Theory, as extended by its author, in proving too much, may be said to destroy itself. Let it be limited to such effects as are fairly deducible from the Alpine phenomena so clearly described by Agassiz, and we must admire in it a vera causa of exceeding interest. But, once pass the bounds of legitimate induction from that vera causa, and try to force the many and highly diversified superficial phænomena of the surface of the globe into direct agreement with evidences of the action of ice under the atmosphere; and you will be driven forward, like the ingenious author of the theory, so to apply it to vast tracts of the globe, as, in the end, to conduct you to the belief that not only both the northern and southern hemispheres, but even quasi tropical regions, were shut up during a long period in an icy mantle."

[Unless, framing hypothesis upon hypothesis,]" we also build up former mountains of infinitely greater altitude than any which now exist, we have no adequate centres for the construction of the enormous glaciers which imagination must create in many regions to account for the phænomena. -The observations of M. Böhtling, (a young Russian naturalist of great promise, but, alas! prematurely carried to the grave,-) give the -result, upon a very grand scale-in the Northern territories of