to that of Edinburgh than to the Devonshire flora: and this is a circumstance of great importance, in relation to the present object, when we consider it in connection with the fact, that, so far as rocks are concerned, this county is totally unlike to the tract around Edinburgh; whereas it is by no means equally dissimilar to Devonshire, which contains a considerable portion of granite, our prevailing rock.

Enough has, I hope, been said to show that the opinion of those who believe the native vegetation to be defined by the rocks is not supported by à priori reasoning or by facts. This view of the matter was derived entirely from my own consideration of the subject; but I am glad that it accords exactly with the opinion of the Rev. J. P. Jones and J. F. Kingston, whose excellent observations I may quote, as they admirably confirm my views:- "With all this variety in the rock strata (of Devonshire), we know of no peculiar vegetable features by which to distinguish one formation from another. The Cistíneæ, Conyza squarròsa, and one or two other plants, seem to affect the limestone; the Clématis also appears to grow more luxuriantly amongst the crevices of that rock than elsewhere; whilst the Iris fœtidíssima and the elm prevail most in red sandstone: still, neither of them, nor any other species, so far as we are aware, is exclusively confined to any particular formation. We may trace the same plants frequently along the whole of our coast line, whether the cliffs are sandstone, limestone, or slate; and, if we choose to push our investigation farther along those of Cornwall, we may still find them in the serpentine formation of the Lizard district, and in the granite rocks around the Land's End."

It is probable that the distribution of vegetables is mainly regulated by climate: a term implying a combination of circumstances, which depend very much upon altitude and latitude.* I presume it would be found that, in general, different floras resemble, or differ from, one another, not with any regard to the rocks, but according as the regions to which they relate are similar or not in altitude and latitude. The same leading rocks, and these but few in number, every where occupy the greater part of the earth's surface; yet, in some climates, how different is the vegetation from that which is found in others! Though the rocks are essentially similar, the vegetation of the mountainous part of Aberdeenshire is

*The researches of Hewett Cottrell Watson, Esq., on the geographical distribution of British plants, have a relevant relation to this second question; as shown in his Outlines, and in his communications to some recent numbers of Jameson's Journal. A notice of the Outlines has been given in p. 265-7. of the current volume of this Magazine.-J. D.

entirely different from that of the rest of the county. It has been already mentioned that the flora of this county is more alike to that of Edinburgh than to the Devonshire flora; while, on the other hand, I am inclined to believe that, were a complete comparison made, the plants of the last-named situation would be found to come nearer to those of the second than to the Aberdeenshire species. Our rocks, however, have been stated to be more similar to those of Devonshire than to the rocks around Edinburgh; to which last, indeed, they are utterly unlike, though the plants of the two tracts have a great similarity. These circumstances I am disposed to explain, simply by this county having, in point of latitude and climate, a greater resemblance to the district around Edinburgh than to Devonshire.

These views might, with advantage, be prosecuted much farther than I can at present carry them. I shall just add that it is clear, did any decided connection subsist between rocks and vegetables, it would be very easy to establish the fact indisputably by a suitable list, as the main rocks of the world are but few indeed. Let, therefore, those who advocate such a connection produce a satisfactory catalogue of the kind alluded to. There may be instances in their favour, I admit: but are these more numerous than the exceptions to every general rule?

It ought not, however, to be considered as certain, that a small number of plants may not show a degree of predilection, or the contrary, for one or more rocks. This may be, in some measure, the case with regard to limestone and serpentine. It is probably true that lime favours the existence and propagation of wild plants: and hence it may be explained why fewer plants are often found on primitive tracts than upon others, as lime is a very sparing ingredient in the oldest rocks. In this way I am disposed to explain a part of our inferiority in vegetable numbers; limestone being more abundant in the two other districts than it is in Aberdeenshire. The difference may also be imputed, in some degree, to our less genial climate: but, indeed, not a little of it may be owing to the Aberdeenshire list being almost entirely the result of my own observation, not always under the most favourable circumstances; whereas, the other tracts have been more fully investigated. It also deserves to be recollected, that all the species found only in the alpine parts of Aberdeenshire are excluded from the list. As to serpentine, I have made the following observations upon a small elevated tract in the parish of Strathdon, composed of this rock. Not only is the rock itself uncommon in that quarter, but its

vegetation is likewise peculiar, entirely different from that of the contiguous hills. These are invariably clothed with the heaths; but the tract just mentioned derives its name (Green Hill) from a covering of grass. It produces Arenària vérna, Cochlearia officinàlis, and Státice Armèria; of which the last two are very rare in that quarter: as to Arenària vérna, I never met with it in that part of Scotland, unless on the spot mentioned; and again in the parish of Leslie, growing over serpentine, the same rock with which it is associated at Strathdon.

Though this communication may be already quite long enough, I wish to occupy a few sentences with a notice of remarks, in this Magazine, upon some observations made by me at a previous period. In the first place, my thanks are due to Mr. Thomson, Manchester, for his courteous and interesting observations, in Vol. III. p. 410-419. Mr. Loudon has likewise, with reference to me, made a few remarks, of a fair and judicious description, in Vol. V. p. 195.; and there (p. 196.) directed my attention to a contributor of a very different stamp, who writes in Vol. IV. p. 460., under the signature of Causidicus. I may be permitted to copy a couple of sentences, as being the origin of the strictures of this writer: -" I venture to suggest, that there is not, in science, a more hopeless labour, than the attempt to discover an unvarying and necessary relation between most plants and the chemical nature of the soils in which they spontaneously grow. I feel convinced that, except in point of moistness, compactness, and depth, soils have in general no cognisable relation to their native products." I shall not offer a detailed notice of the remarks of Causidicus, but content myself with requesting your readers to refer to them; and I shall be satisfied that the cause be judged upon his own pleading. I may, indeed, just quote his opening sentence relative to me. "The first point whereon I should differ from Mr. Thomson would be, the apathetic calmness, and almost scepticism, with which he treats that most extraordinary hallucination of Dr. Alexander Murray, which prefaces Mr. Thomson's remarks." This passage (which alludes to the opinions above quoted) may be considered a pretty fair specimen of the whole article. The meaning of the sentence no one can positively tell; and the meaning which it may be guessed to have is entirely devoid of foundation, as Mr. Thomson's allusion to me is of a complimentary description. With respect to the "hallucination," I shall not say much on that head, as no one can be considered the best judge of whether or not his own opinions deserve this epithet. I shall, however, observe that

the soundest thoughts may pass for error and "most extraordinary hallucination," in the view of those whose information or judgment happens not to be of a first-rate description. It may suit the quibbling turn of mind occasionally imputed to the profession to which the signature of Causidicus implies him to belong, to allege, as he does, that "Dr. Murray entirely changes the terms of his proposition. There is a vast difference between a decided connection' and an unvarying and necessary relation.'" I shall only add that Causidicus is welcome to view me as standing upon either of the positions which he appears to have in his eye; and I am persuaded the footing is firm enough to enable me to defy such efforts as his. I am, Sir, yours, &c.

Aberdeen, April 17.1833.

ALEXANDER MURRAY.

THE above communication relates strictly to the connection between rocks and plants. On the relation between soils and plants, see Mr. Goldie, in Gard. Mag., vol. ii. p. 125.; Mr. Gordon, in vol. iv. p. 463.; Causidicus, in vol. vi. p. 216.; Mr. Gordon, in vol. vi. p. 359.; Causidicus, in vol. vii. p. 371., vol. ix. p. 291.-J. D.

ART. VIII. Volcanoes. By W. M. HIGGINS, Esq. F.G.S., Lecturer on Natural Philosophy in Guy's Hospital, and J. W. DRAPER, Esq.

(Continued from Vol. V. p. 637.)

GEOGRAPHICAL POSITION AND HISTORY OF ACTIVE

VOLCANOES.

Africa.-THE continent of Africa does not offer a single example of an active volcano; but nearly all the neighbouring islands are of volcanic origin, and many of them present examples of active vents.

The whole group of the Canaries is the product of some vast subaqueous volcano, to the furnaces of which the various islands seem but as openings.

Teneriffe is the first of these to which our attention is drawn. The Peak of Teneriffe is one of the most elevated volcanic mountains, and lifts its towering head to a height of 12,176 feet above the level of the sea. It is a pyramidal mass, having a base of more than 115,110 yards in circumference. The cone is small in proportion to the mountain, and is said to be not more than 537 ft. high. Its crater is about 300 ft. in its greater diameter, 200 ft. in its lesser, and 100 ft. deep. It is not, however, from this that the

eruptions proceed, for it has not been active since the island has been inhabited. Lateral eruptions have not been unfrequent, and particularly from the mountain Chahorra. In the year 1706, an eruption destroyed the harbour of Garachico, which was the finest harbour in the island; and at that time the lava was observed to travel at a rate of nearly three miles an hour. The last eruption from the crater of Chahorra was in 1798, when it threw out lava and scoriæ for the space of six months.

In the Island of Palma there is a conical mountain, the crater of which is 5000 ft. deep; it was in a state of eruption in 1677.

Lanzerote has also been the site of most terrific volcanic action, its last eruption continuing from the 1st of September, 1730, to the 16th of April, 1736. Von Buch has given an interesting description of the phenomena which attended and followed this violent effort, and also a description of the vent itself. The crater is walled in by precipitous rocks; and, in the interior, two other craters are observed. A space of three square miles on the west is covered by a mass of black lava : but the crater was not the only source from which this vast mass of lava was emitted. "How much was I astonished," he says, "when, on reaching the summit, I perceived an entire series of cones, all nearly as lofty as the Montagna de Fuego, placed so directly in a line, that the nearest covered the farthest one in such a manner that their summits alone were seen peeping from behind.”

The Island of Ferro has a volcano, which was active, in 1692, for a continued period of six weeks.

The only island among the Cape de Verde group that has an active volcano is that of Fogo, or Fuego; but, as little is known concerning it, we may turn our attention to the Azores; all of which are of volcanic formation, and some contain active volcanoes.

M. Bory de St. Vincent has given a drawing of Mount Dolomieu, in the Isle of Bourbon. (fig. 45.) This cone is one of those in the phase of permanent activity. It has no regular crater; but occasionally opens small apertures as required, and fills them again during the continuance of the eruption. The table-formed appearance of the top of this mountain furnishes us with a good example of the manner in which lava may, in the first instance, have issued from fissures in the plain surface of the earth before the mountain mass itself was elevated.

St. Michael's Island, which is the largest of this group, has several times suffered from volcanic action. The last eruption