an enormous scale, which the rocks in contiguity to the deep-seated igneous masses have suffered, and, probably, are still undergoing.*

25. REVIEW OF THE LOWER PALEOZOIC SERIES.-In conclusion, I will briefly review the leading phenomena which have been brought under our notice in the course of this Lecture.

The strata comprised in the Silurian or Cambro-Silurian series present all the usual characters of marine sedimentary deposits. The fossils comprise traces of marine worms throughout the series, many corals, chiefly in the upper limestones, numerous crinoids, immense numbers of extinct forms of the lower crustacean tribes, and of brachiopodous mollusca, with many of the lamelli-branchiate, gasteropodous, pteropodous, and cephalopodous families. The fishes come in late in the series, together with scanty relics of terrestrial plants. The marine vegetation of the period is indicated only by obscure fucoidal markings, and possibly by the occasional anthracitic and bituminous bands of the lower rocks. These organic remains belong, for the most part, to peculiar generic types, mostly ranging throughout the fossiliferous Cambro-Silurian strata, and some extend into the upper palæozoic formations, but none occur in the secondary deposits.

In fine, the lower paleozoic rocks (the," Silurian and Cambrian," of Sedgwick, or "Upper Silurian, Lower Silurian, and Bottom-rocks," of Murchison) have resulted from marine depositions, going on through immense periods of time, in seas swarming with zoophytes, crinoids, crusta

The papers by Sedgwick and D. Sharpe, on Cleavage, are to be found in the Geolog. Transact. and Journ. ; Mr. Sorby's papers in the Phil. Mag. and Edinb. N. Phil. Journ. Their researches, together with those of Darwin, Hopkins, Tyndall, Rogers, Haughton, and others, are admirably collected and illustrated by Prof. Phillips, in his "Report on Cleavage," to the Meeting of the British Assoc. 1857. See also Sir R. Murchison's concise résumé of the facts and hypotheses relating to the interesting subject of Cleavage, in "Siluria," 2nd edit. chapter 2. For some remarks on Metamorphism of Rocks, see further on, Lect. viii. part 2.

ceans, and molluscs, and ultimately with fishes. Much of the lower portion of this great sedimentary series has been subjected, since its original deposition, to metamorphic influences, by which the sedimentary characters have been either greatly modified or entirely obliterated; still some of the "bottom-beds" (Longmynd) are even less altered than great portions of the succeeding groups of Llandeilo and Caradoc.

Some eminent geologists are of opinion, that there is a line in the descending series of strata, where organic remains entirely disappear; and that this line is by no means coördinate with mineral changes induced by igneous action. As regards the absence of fossils in the porphyriferous schists of Llandeilo age, it may be in part attributable to the obliteration of all vestiges of organic remains, in consequence of the high temperature to which they have been exposed; or animals may not have been capable of living in an ocean subjected to continual incursions of igneous matter. But the bottom-rocks are sometimes not so much altered but that fossils might be detected in them, if present; and this, to some extent, has been put to the proof, at Bray Head and the Longmynd, by Dr. Kinahan and Mr. Salter,* who have thus extended our knowledge of primeval animated nature into periods once thought to be "azoic." What further glimpses into the natural history of the primordial regions we shall obtain, the persevering and active researches of intelligent collectors will alone show.

With the Oldhamia of Ireland, the Palæopyge of the Longmynd, and their associated Arenicolites, we lose at present all positive evidence of the presence of organic beings on the surface of the primordial earth; but it would be rash to assume that these most ancient fossils are the relics of the earliest living things that tenanted our planet. Well has Sir C. Lyell remarked, that "it is too common a fallacy to fix the era of the first creation of each tribe of plants or animals, and even of animate beings in general, at the precise point where our present retrospective knowledge happens to stop." †

*See above, p. 808.

† See also "Travels in North America," vol. ii. p. 128, &c.

LECTURE VIII.

PART II.-THE VOLCANIC AND HYPOGENE ROCKS.

1. Introductory. 2. Nature of Volcanic Action. 3. Earthquakes. 4. Earthquake of Lisbon. 5. Phlegræan Fields, and the Lipari Isles. 6. Ischia and Vesuvius. 7. Structure of Volcanic Mountains. 8. Volcanic Products. 9. Lava-currents, Dykes, and Veins. 10. Eruptions of Vesuvius. 11. Herculaneum and Pompeii. 12. Conservative effects of Lava-streams. 13. Organic Remains in Lava. 14. Mount Etna. 15. Val del Bove. 16. Volcano of Kilauea. 17. Mr. Stewart's visit to Kilauea. 18. Volcano of Jorulla. 19. Submarine Volcanos. 20. Summary of Volcanic Phenomena. 21. Hypogene Rocks. 22. Mica-schist and Gneiss. 23. Contorted Crystalline Rocks. 24. Basalt or Trap. 25. Isle of Staffa. 26. Strata altered from contact with Basalt. 27. Trapdykes in the Isle of Sky. 28. Granite. 29. Granitic Eruptions. 30. Metamorphism of Rocks. 31. Precious Stones. 32. Metalliferous Veins. 33. Auriferous Alluvia. 34. Cupreous Deposits. 35. Transmutation of Metals. 36. Review of the Hypogene Rocks. 37. Organic Remains in metamorphic Rocks. 38. Chronology of Mountain-chains. 39. Systems of Elevation. 40. The Great Caledonian Valley. 41. Structure of Ben Nevis. 42 Retrospect. 43. Successive changes in the Organic Kingdoms. 44. Geological effects of Dynamical and Chemical Action. 45. Strata composed of Organic Remains. 46. General Inferences. 47. The Ancient World. 48. Corollary. 49. Final Effects. 50. Concluding Remarks.

1. INTRODUCTORY.-We have now passed the boundary which separates the animate from the inanimate world, and have entered those regions of geological research in which all traces of organized beings are absent, and various modifications of mineral substances are the only objects that meet our view. The rocks no longer exhibit those organic characters by which we were enabled to decipher the natural records of the past, entombed in the fossiliferous strata; but they are inscribed with hieroglyphics whose meaning is often obscure and frequently unintelligible, and many of which admit of a double interpretation.

The mechanical effects of water as a fluid, everywhere so manifest in the sedimentary formations, are no longer appa

rent; but the powerful agency of the same substance in the several conditions produced by high temperature-as vapour, steam, and gas *-is universally exhibited in the upheaved mountain-chains, the protruded igneous matter, the rent and dislocated rocks, and the rugged peaks and precipitous glens, which are the characteristic features of the physical geography of schistose and granitic regions.

In the phenomena attendant on the earthquakes and volcanos of modern times, we have proofs of the continued energy of those physical forces which produced the results that will form the principal subjects of this discourse. The nature and effect of volcanic action (to which we have previously and incidentally alluded) now, therefore, demand our especial consideration, in order that we may comprehend the origin and formation of the metamorphic and plutonic rocks; or in other words, of those mineral masses which have acquired a crystalline structure from exposure to the influence of intense heat under great pressure, and have been elevated into their present position by subterraneous movements. I propose, in the first place, to consider the nature and effects of igneous action as exhibited in existing volcanos; secondly, to describe the hypogene or plutonic rocks, and the changes produced in contiguous mineral masses by their influence; and, lastly, to offer a few general remarks on some of the most important subjects that have engaged our attention.

2. NATURE OF VOLCANIC ACTION.† — Volcanic action

The beautiful experiments of Mr. Grove prove that water may be reduced to its elementary compounds-oxygen and hydrogen-by intense heat alone. See Philos. Trans. 1846.

+ Daubeny, Darwin, Scrope, and Lyell are chief among the English geologists who have made the causes and effects of volcanos a special study. The Transactions and Journal of the Geological Society are enriched with several of their Memoirs; and the special works of Scrope and Daubeny on Volcanos ought to be in the hands of the student. In the appendix to the second edition of Dr. Daubeny's comprehensive and invaluable "Description of active and extinct Volcanos, of Earthquakes,

is referred by Humboldt to the influence exerted by the internal heat of our planet on its external surface; by which concussions of the land, or earthquakes, and the expansion or elevation, and the subsidence or contraction, of large portions of the solid crust of the earth have been produced. The number of existing volcanos is estimated at about 200, of which 116 are situated in America or its islands.

In the previous discourses, many of the effects of igneous agency came under our notice,* namely, the subsidence and elevation of the Temple of Serapis (p. 106), the gradual rise of Scandinavia (p. 116), the upheaving of the sea-coast of Chili (p. 112), and other mutations of a like nature. As we successively examined the tertiary, secondary, and paleozoic or primary formations, proofs that similar phenomena had taken place during every geological epoch were equally manifest; the geographical distribution of the foci of volcanic action was found to have varied, but throughout the cycle of physical changes contemplated by Geology, the volcano and the earthquake appear to have been in ceaseless activity.

and of Thermal Springs, &c.," will be found a full and classified list of British and foreign works on the same subjects previous to 1848. The first volume of Humboldt's inimitable "Cosmos" has a section devoted to the subject of Volcanic phenomena, and supplies many references to Von Buch, Arago, Bischof, Prévost, Gay Lussac, and other continental authors. Abich, Deville, de Beaumont, Dolomieu, and Scacchi are other European geologists who have written valuable treatises on Volcanos; and Dana heads the list of American volcanists. Earthquakes and their associated phenomena have necessarily been treated of by many of the above-named geologists; but M. A. Perrey, of Dijon, and Mr. Mallet, of Dublin, have especially taken up the subject, in some of its aspects; the results of the researches of the latter will be seen in the Reports of the British Association; the publications of M. Perrey may be found in the Memoirs of the Scientific Societies of Paris, Lyons, Brussels, &c. The student is particularly recommended to read the chapters on Volcanic Rocks and Action, in Lyell's " Manual of Geology," 5th edition.

* A concise view of the effects of high temperature and of volcanic action on the earth's crust was given in the first Lecture, vol. i. p. 103.