THE

GEOLOGICAL MAGAZINE.

NEW SERIES. DECADE II. VOL. V.

No. V.-MAY, 1878.

ORIGINAL ARTICLES.

I. NOTES ON THE DEVONIANS AND OLD RED SANDSTONE OF NORTH AND SOUTH DEVON.

IT

By A. CHAMPERNOWNE, M.A., F.G.S.

(PLATES V. AND VI.)

T may seem to many geologists, accustomed to speak of the simple order of North Devon," a somewhat unusual process to fall back on the more troubled districts of the southern division of the County for evidence upon which to take one's stand in interpreting the grand facts of succession within the original "Devonian" area, Devonshire. That is, however, the plan of campaign which I shall adopt; but instead of publishing the paper I read last year before the British Association, in Section C, at Plymouth (which will appear in abstract in the Annual volume of Reports), I will only recapitulate the facts I then brought forward, relating to the southern limestones of Torbay.

In doing so, my chief reason is that I have heard the value of that evidence denied, and the original reading of the section strictly adhered to.

In the two diagrams on Plate VI. Figs. 1 and 2, (1) represents the limestone of Berry Head, and the flap of the same south of Mudstone Bay; (2) the slates of Mudstone Bay; (2) the slaty rocks of a slightly different aspect south of the limestones; (3) red sandstones corresponding to those of Staddon Heights and Mount Edgecumbe ; (4) intrusive trap altering the rocks at the contact; (d) hæmatite iron-ore deposited in the hollows of the limestone, and not a bed, as sometimes represented.

It will be observed that (10) is a trough of the limestone beds, the bottom of which is clearly seen; close beyond it unites with the main mass. The right-hand tongue of trap forms the extreme headland of Sharkham Point, the limestone adjoining it on the right being nearly vertical, whereas it sets on at the southern end of Mudstone Beach at an angle not greater than 45°.

Now I contend that in the minor trough which we can see, we are furnished with a direct clue to the structure of the main mass, the bottom of which is not seen, but is at some depth below the sea. I believe that such dotted lines as those in diagram (1) are natural, but that those of diagram (2) are scarcely conceivable. The minor trough cannot be ignored, and I cannot imagine it possible that a portion of the slates (2) can have been introduced

DECADE II.-VOL. V.-NO. V.

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between the two parts of limestone if they (the slates) are really newer beds. Therefore I conclude that they are not newer beds, but that all the rocks on the left of the diagram are older than the limestones. This brings the Red Sandstones (3) into direct and natural relation with those of Cockington, the Warberry, Lincombe, etc., at Torquay, which are beneath the limestones.

I may add that thick-bedded red sandstones on the strike of (3) in the Halwell Valley, nine miles to the westwards, dip north, the Harbertonford limestone also dipping north, though thrown by a N. and S. fault on its east; and I believe that as soon as the boundaries shall have been accurately surveyed, the real infraposition of the red sandstones will be established beyond doubt.

Now, having at Plymouth received from my opponents, of whom not the least formidable was my friend Mr. Pengelly, our Sectional President, a sort of challenge to study North Devon, about a month later I took the hint, especially as I had not visited that part of the country since the time when I held opinions widely differing from those I now believe to be true.

I first went to the north side of Morte Bay to see the Morte slates close to their junction with the Pickwell Down sandstones, and I give my notes almost as I took them.

Between Barricane Beach and Woolacombe the Morte slates dip at 65° and 70° S., crossed by cleavage dipping north at about the same angle; the latter dip was obviously not due to "surface bending," though that is sometimes seen in the neighbourhood. The reefs are a mass of jagged edges formed by the two sets of planes. Some very even quartz veins stand in the plane of the cleavage, and also vertical.

The slate is here distinctly bedded. Some beds are coarse and brownish, semi-arenaceous. One bed of brown sandstone 3 ft. 6 in. thick, obliquely traversed by quartz veins, is just like the Ilfracombe, etc., grits. With these exceptions, the character which the Morte slates bear of "fine-grained, smooth, glossy slates," will apply to them very close to the junction.

Still nearer Woolacombe the beds bend over as they rise, evidently forming part of a great denuded curve, as shown in the accompanying sketch, Plate VI. Fig. 3. The spot is about 200 paces from the foot of the blown sands, as near as I could take it for the advancing tide. Still nearer the sand hills the beds dip 80° S.

This curve, if not large enough singly, forms probably one of a series of curves which would carry the beds right over Pickwell Down, bringing them in again on the south.

Where the brook cuts through the sand hills at the Life-Boat House are some very smooth slates belonging to the Morte series. About 600 paces south of the Boat House is a reef of grey and purplish sandstones and slates dipping S. 65°. These should probably be included in the Old Red Sandstone; the boundary-line at any rate must be drawn somewhere about the foot of Potter's Hill.

The tide being almost full, I was unable to see much of the reefs which, according to Mr. Etheridge, are very well shown; but as long