It must be understood that the above section refers to the Force itself, the actual place where the water comes over. The sections on either bank of the Tees a few dozen yards below the Force differ from that at the Force itself, and at the same time differ inter se.

On the north bank, at about 20 yards below the Force, it can be distinctly seen that there is no separate bed c, and that the space between the top of a and the bottom of c is one uniform mass of basalt. We have, then, the following as the section:

Basalt, 32 feet (+ any that may be above the level of the top of the basalt at the Force).

Shale, 12 feet.

Limestone, 32 feet.

We will call this section the section at X.

On the south bank the section, at a point about 30 yards below tho Force, is:

Basalt, 24 feet (+ any that may be above the level of the top of the basalt at the Force).

Shale, 20 feet.

Limestone, 32 feet.

We will call this section the section at Y.

Owing to the banks being, in places, inconveniently steep, and bearing a thick covering of hanging trees &c., the entire passage between the above three sections cannot be quite so well seen as one would wish; enough, however, is shown to enable one to come to a safe conclusion as to the true state of the case. On looking at the section at the Force more carefully, it is distinctly seen that the bed of shale, b, is thicker on the south side than on the north side of the Force that from 14 foot it dwindles down to one foot, and from one foot it dwindles more and more as we go towards the section at X, until it is only doubtfully represented by a thin line marked by ferns and grass, and that this thin line then ceases, and we get the state of things at X-that, on the other hand, as we go from the Force towards the section at Y, the bed of shale, b, gets thicker and thicker (from 1 foot it gets to 2 feet, and so on), until, at a point about 20 yards below the Force, it is 3 feet thick, and in 10 or 12 yards more it has thickened out and formed one mass with the bed of shale, d; and now there is no underflow of basalt.

It appears, then, quite safe to conclude that the underflow of basalt, which has its origin from the main mass at X, gradually thins out as we go south-west and south from this point, until at Y it has ceased altogether.

If we examine the bank a few yards beyond X, we find the section change again, and again we have an underflow of basalt separated from the main mass by a thin bed of altered shale, thus:

Basalt.

Sandy shale, about 2 feet.

Basalt.

The top of the underflow of basalt here is roughly on the same

level as that of the underflow at the Force, and the shale parting, which at first makes its appearance somewhat abruptly, can be seen for a dozen yards or so gradually thickening as we follow the bank of the river in a north-easterly direction. The section then becomes lost, owing to the covering of tumbled whin, &c., and we have to go on for some 70 yards before the beds are well shown again. Here the section is much the same as that at Y, viz.:

Basalt.

Shale, about 20 feet.

Limestone.

We have, then, good evidence that the section at X changes towards the north-east in the same way as it does towards the southwest and south.

And so, finally, we see that near X a small part of the lower portion of basalt becomes separated off from the main mass, and that this lower portion rapidly thins out towards both the south-west and south, and the north-east.

There are other places, both in the immediate neighbourhood of the High Force and elsewhere, where similar undoubted underflows exist. Sometimes these underflows can be seen distinctly in the act of being gradually separated off from the main basalt mass, altered beds coming in between; and sometimes these altered beds consist of altered sandstone, or of altered limestone, instead of altered shale. And, as there are underflows, so also there are overflows; parts of the top of the basalt gradually get separated off from the main mass, and then thin out.

In some cases we can see beds of altered shale and sandstone apparently surrounded by basalt on every side: there is basalt above; and there is basalt below; and the altered beds can be seen to end in basalt as you follow the section along on either hand. There is a very good instance of this in the Slate Scar, a little more than half a mile above the High Force. The greatest dimensions of these beds are seen to be generally in the direction of their planes of original deposition; their thicknesses are, in fact, generally strikingly less than their length. For instance, with an average thickness of 2 feet, we may get a length of 60 yards. This is, I think, strongly suggestive of some stable connexion between these and the other beds among which the basalt has been intruded; they appear to have kept something of their original position very much better than one would suppose at all likely if they had been completely isolated in the great basalt mass. I would suppose, then, that, certainly in most cases, these beds are not surrounded by basalt on every side-have not, in fact, been simply "caught up" in it--but that, if they could be traced in directions both away from the observer and towards him, they would, in one of these directions, be found to join on with the great mass of the sedimentary rocks of the country. It is evident that, in any ordinary section, we have only four sides visible out of the six wherewith any geographical point may be supposed

may

to be bounded; and in either of the two unseen the state of things be very different from that seen in the others. These subordinate beds of basalt have been spoken of as underflows and overflows; in every case observed, however, they thin out so very rapidly that it might be better to call them simply wedges. They appear to belong in every way to the main mass of basalt; and probably one of the ordinary methods of the thinning out &c. of the basalt is the giving off of these separation masses, and their then dying out. Thus the thinning out &c. of the basalt as a whole might probably be best represented, diagrammatically, somewhat as below:

Fig 2.-Thinning-out of the Busalt at the High Force.

I do not wish to imply by the above diagram that there is one point from which the basalt can be made out to be thinning away in every direction, but simply that, where the basalt is thinning out, the thinning-out probably takes place in some such way as represented.

6

Prof. Sedgwick, in his paper on the Geology of High Teesdale (Cambr. Phil. Trans. 1824), brought forward evidence which was quite conclusive as to the truly intrusive character of the whin sill. Prof. Phillips, at the time he wrote his Geology of Yorkshire,' does not seem to have loyally accepted this conclusion, but to have been more impressed with its supposed "general conformity over a vast area to the beds above and below." It is this partial conformity which still causes it to be an article in the miner's creed that the great whin mass is a sill," a bed regularly interbedded with the other beds or "sills." On close examination of the ground, however, it is seen that this conformity is not nearly so general as is supposed, and that even in cases where the whin, as a whole, does keep on the same geological horizon, there are numerous instances of small subordinate beds separating off from the main mass in the manner already described, and conclusively proving its intrusive character. The High Force is one of the four localities specially mentioned by Phillips (Geology of Yorkshire,' part ii. p. 77) as showing the conformity between the surfaces of the whin sill and the surrounding beds. We have seen that, instead of showing this, it affords striking evidence to the contrary. The bed of the Tees near Winch Bridge is another of the localities mentioned. This, too, shows good evi

dence to the contrary; and I have very little doubt that the more the country is worked over the more and more evident will the intrusive character of the whin sill become.

P.S. Since the above was written, Mr. F. Rutley has kindly cut, and microscopically examined, sections of the rocks a, b, and c of the High-Force section (vide suprà); and the results obtained entirely confirm my statements: a and c are basalts; and b is evidently an altered sedimentary rock. The examination of the rocks in the field really left no doubt on this head.

52. On the MECHANISM of PRODUCTION of VOLCANIC DYKES, and on those of MONTE SOMMA. By R. MALLET, Esq., C.E., F.R.S., F.G.S. (Read June 21, 1876.)

In the year 1864 I was enabled to employ some time in the study of some of the chief volcanic phenomena presented by the cone of Etna, devoting my attention principally to the laws which govern the flow of lava currents, the formation of the parasitic cones which abound upon certain portions of the surface of the great mountain, of which Monte Rosso and Monte Peleri in the neighbourhood of Nicolosi are amongst the largest examples, and in examining the Val del Bove and the many so-called dykes which intersect its surrounding escarpment in so many places. These last objects were examined by me with the able memoir of Sir Charles Lyell, published in the Philosophical Transaction for 1858, in hand, the title of which is "on Lavas of Mount Etna," &c.

In the second part of that memoir the author rests part of the evidence upon which he concludes that an ancient great vent existed in the Val del Bove at Trifoglietto, upon the convergence at about that point of the prolonged lines of direction of thirteen dykes existing in the surrounding escarpment, in accordance with the views previously promulgated by Von Waltershausen (Phil. Trans., Part ii. 1858, page 703). I have no intention of casting any doubt here upon any part of the above able memoir of Sir Charles Lyell, in all the main conclusions of which, indeed, I concur, my present object being limited to remarking upon some of the conditions which affect the formation of volcanic so-called dykes of injection, such as those existing in the escarpment of the Val del Bove, and of that of Monte Somma, and to pointing out the extent to which the orientation or direction of such dykes can be safely employed as a means for determining, with any thing approaching to certainty, the existence or position of a central chimney or crater, from which such dykes may be supposed to have emanated, by means of the intersection of lines or planes prolonged in the direction of such dykes. On going round nearly the entire escarpment of the Val del Bove, with Von Waltershausen's map in hand, the dykes, which are almost innumerable, when examined at many points both from above and below the escarpment, presented phenomena in many other respects as well as in direction, so perplexing as to raise in my mind much doubt as to whether they could be at all accounted for upon the commonly accepted theory of their production, and suggested the inquiry whether that theory which assigns their production to the injection of fissures in crater walls by the liquid lava within the crater might not need considerable modification, and if so, whether, in its imperfect state, the orientation of dykes could, by intersection, be safely employed at all for determining the position of craters which have disappeared, as in the case of that of Monte Somma