pterygoid shows two kinds of dentition; on its outer edge is a single row of twenty-six large teeth of equal size, which are somewhat smaller than those of the maxillary bone. Internal to these on the anterior part of the pterygoid are from three to six irregular rows of short pointed teeth similar to those on the parasphenoid. In the under-jaw there are thirty teeth.

This instalment of Dr. Fritsch's work leads us to look forward to its subsequent progress with great interest. It is a monument of conscientious labour and admirable skill in deciphering remains in which the characters presented more than ordinary difficulty. H. G. SEELEY.

CORRESPONDENCE.

THE PURPLE BOULDER-CLAY AT HOLDERNESS.

SIR,-In the excellent account of English glacial deposits, given in the second edition of "The Great Ice Age," it is stated that “in Holderness the purple clay is quite unstratified." It is true, that in many places the "Purple Boulder-clay" is unstratified; but in many others it is distinctly stratified. The stratified and unstratified portions are, however, so mixed up together, and run so into one another, as to be quite inseparable. The deposit has also been much shoved about, the thrust having in some well-marked cases the N.N.E.

DRIFFIELD, Oct. 9th, 1879.

come from

J. R. DAKYNS.

ON THE CLASSIFICATION OF THE BRITISH PRE-CAMBRIAN ROCKS.

SIR, Will you kindly call attention to the fact that by some mistake the word not has been left out after the word did in my paper, p. 434, line 19 from bottom of page. The sentence should read, "If these movements also did not take place, etc." As the absence of this one little word does away with the proper meaning, I shall feel obliged if you will kindly notice it in the next Number of the GEOL. MAG. HY. HICKS.

HENDON, N.W., Oct. 3, 1879.

SURFACE GEOLOGY OF THE MISSISSIPPI VALLEY.

Mr. W. J. Mc Gee writes from Farley, Iowa, September 18, 1879, in reference to his communication, which appeared in the GEOL. MAGAZINE for August, pp. 353-362, and September, pp. 412-421 :— "Thanks for your care in revising proofs of my paper in the GEOL. MAG. A very few errors have crept in, however, which I will note. W. J. Mc GEE."

Page 358 (August No.), line 30, for "mentioned" read" weathered."

COLUMNAR SANDSTONE IN SAXON SWITZERLAND.

We are requested by Mr. Walter Keeping to make the following corrections in his article which escaped notice-namely, at p. 438, line 2 from foot, for binding' read bending, and on p. 441, line 8 from top, for Yorkshire,' read 'Derbyshire.'EDIT. GEOL. MAG.

THE

GEOLOGICAL MAGAZINE.

LA

NEW SERIES. DECADE II. VOL. VI.

No. XII.-DECEMBER, 1879.

ORIGINAL

LIBRAR

UNIVERSITY

ARTICLES.

'ALIFORNI

I. THE PARALLEL ROADS OF GLEN ROY,

By J. R. DAKYNS, M.A.;

of H.M. Geological Survey of England.

AST summer I paid a visit to the Parallel Roads of Glen Roy, and saw certain features which must, I think, have hitherto escaped notice. It has been asserted that the roads consist of, or are cut out of, mere superficial detritus; and that they never appear where the solid rock appears; and much discussion has arisen as to the mode of formation of the roads on this supposition, viz. that they consist of detritus merely.

In order to get a section of the roads, I examined several watercourses descending the side of Leana Mhòr, west of the River Roy. These water-courses were cut to the depth of about ten feet in the ordinary superficial detritus of the mountain side; and where they crossed the roads, the material exposed on their sides was precisely the same as that along the rest of their course. This did not surprise me, as I had recently read in the GEOLOGICAL MAGAZINE for July (p. 321), in a notice of a paper by Professor Prestwich, "On the Origin of the Parallel Roads of Lochaber," that "the Parallel Roads are terraces composed of perfectly angular fragments of the local rocks." However, as these water-courses nowhere reached the solid rock, I was not satisfied. I accordingly went next day to the east side of the River Roy, where the roads are very well marked indeed, and are crossed by two or three comparatively large gills. These gills descend along the west face of east Leana Mhòr; for there are two hills of this name given on the One-inch Ordnance Map, one on the west, and the other on the east side of the River Roy.

I walked along the topmost road northward; and the first gill I came to showed me at once that the road was cut out of the solid rock. Both above and below the road the solid rock came practically to the surface, being covered with a mere film, about a foot thick, of its own detritus: the surface along the road itself was, however, hidden by a small fan of detritus shot on to the road from the gill above. The section of the second or middle road was obscure along this gill. I walked on along the highest road till I came to a big gill cut deeply into the native rock, and bifurcating a little below the 1250 contour line. The highest road crosses this pair of gills above the point of bifurcation, and was so utterly destroyed as to be quite obscure; but the second road ran right up to the edge of the

DECADE II.-VOL. VI.-NO. XII.

34

deep gill, which it meets just below the junction of the two branch gills, and started at once again on the opposite side; no talus concealed the road; it is cut out of the solid rock. A subordinate road which occurs a little further north between the two ordinary top roads was also seen to coincide with and, in fact, to be a rock feature. I was satisfied: I went no further: I had had two clear sections of the two best-marked roads, one of each: the roads are, at all events in some cases, cut out of the solid rock. It is reasonable to conclude that where they appear to be confined to superficial detritus, it is because the rock shelf is hidden by said detritus, which has, in fact, accumulated along the road just because it is a shelf. It is noteworthy that whereas the roads on the east side of Roy, where being free. of detritus they are seen to form rock shelves, are wide enough to allow two carts to pass each other. On the west side where encumbered with debris they are no broader than a good sized foot-path.

The roads, though sensibly parallel and horizontal, are not absolutely so; for, according to the measurements of the Ordnance Surveyors, the first road in Glen Roy varies in elevation from 1144 to 1155 feet above the sea, the second from 1062 to 1077, and the third from 850 to 862. Such variation in height would necessarily be caused by the unequal accumulation of debris on the original shelves.

It is right to say that there is an error in the mapping of the roads on the One-inch Ordnance Map, Sheet 63: on the map the topmost road is represented as crossing the big two-grained gill just below the bifurcation; it really is considerably higher up, while it is the second or middle road which crosses the gill at the fork.

Diagram showing the profile of the mountain side with the three roads, the two highest, Nos. 1 and 2, well marked, and the lowest, No. 3, not so well marked, as seen by an observer looking up Glen Roy.

I have not seen the places where, as Mr. Darwin' observes, the shelves entirely disappear on crossing the part of the mountains in which the base-rock is exposed; but I would ask whether the rock 1 Quoted by Sir John Lubbock, Quart. Journ. Geol. Soc. vol. xxiv. p. 84.

may not be more than usually tough in these places. Generally the rock seemed to me to be of a character easily disintegrated.

How then were the roads formed? Obviously by the planing action of waves acting on the rock and cutting the shore-line back. Subsequently, on the lowering of the water-line, the detritus of the mountain side falling, and being washed down hill by rain, etc., lodged on the platform of the shelves, and there accumulating gradually formed a pile of loose material sloping towards the valley, and slowly obliterating the true roads.

I add a sketch of the profile of the mountain side, as seen from the big gill looking up the valley. It will be seen that my profile is quite similar to the diagram given from Macculloch by Sir John Lubbock in the Q.J.G.S. for 1868, vol. xxiv., with this remarkable difference in my sketch the two highest roads are represented as comparatively near to one another, while the lowest is much lower down; this is the case in nature; the average heights of the roads above sea-level being, according to the Ordnance Surveyors, 1148, 1067, and 855 feet; but in Sir John Lubbock's diagram, after Macculloch, the two lowest roads are represented as close together, and the third much higher up: the diagram has evidently been drawn or printed upside down; for turn the book topsyturvy and the diagram is right.

As to the nine points then, on which, according to Sir John Lubbock, we have a substantial agreement, the whole question of the true nature of the roads turns upon the second and third: these I would controvert thus: the horizontal roads are shelves cut out of the solid rock; and these shelves only appear when the solid rock itself appears, being in other cases entirely hidden by the debris of the mountain side, which has fallen and accumulated upon the shelves.

It is further to be noted that the disintegration of the rock forming the shelves would in course of time disguise the fact that the roads consisted of rock shelves, and cause them to appear as mere heaps of detritus.

II. DESCRIPTION OF A NEW SPECIES OF FOSSIL FISH SPINE, CTENACANTHUS MINOR, FROM THE LOWER COAL-MEASURES YORKSHIRE.

A

By JAMES W. DAVIS, F.L.S., F.G.S., etc.;

OF

Hon. Secretary of the Yorkshire Geological and Polytechnic Society. SHORT time ago, whilst examining and appending the names to a number of specimens of fossil fish in the collection of the Bradford Philosophical Society, which are being arranged and exhibited in the new Corporate Museum in that town, I came across the spine which is the subject of this notice. It is from the Blackbed Coal at Dudley Hill, near Bradford. It was associated with a number of fish remains, amongst others :

Ctenacanthus hybodoides, Egerton.
Gyracanthus formosus, Agass.

Pleuracanthus lævissimus, Agass.

Diplodus gibbosus, Agass.

Ctenoptychius, sp.

Petalodus Hastingsiæ, Agass.
Acanthodes, sp. ?

Megalichthys Hibberti, Agass.
Coelacanthus lepturus, Agass.

Rhizodus Hibberti, Agass.

Several specimens of Labyrinthodonts, in excellent preservation, have been found along with the remains of the fishes. One of them, a new genus of large size, was described and named by Professor Huxley Pholiderpeton scutigerum.

The Black-bed Coal is in the Lower Coal-measures about 120 feet higher in the series than Elland Flag-rock. Immediately above the Coal there is a bed of shale containing a considerable quantity of Clay-ironstone in nodules, which is worked by the Low Moor Iron Company along with the Coal. The fossil fish and Labyrinthodonts are from this shale immediately above the Coal.

Ctenacanthus minor, Davis, sp. nov. (Natural size.)

Ctenacanthus minor, mihi. Spine: length, 14 inches; greatest breadth, 3 of an inch. The base is imperfect, and the tip of the spine is also wanting. It is slightly curved, is very strong, and has been very deeply implanted in the flesh; the basal portion constitutes more than half the entire length, and if the specimen had been perfect would have taken up quite two-thirds; probably a larger proportion than exists in any other species of Ctenacanthus. The lateral faces are compressed anteriorly; posteriorly they expand and give to a section of the spine a triangular form. Along the back of the spine there is a deep groove, no posterior denticles can be seen. The line dividing the exposed part of the spine from the base forms an angle to the length of the spine of about 45°. The anterior portion is produced so as to form a median keel, whilst on each side there are six or seven well-defined ridges or costæ separated by deep intercostal spaces. The ridges run parallel to the posterior margin for the most part, whilst those near the anterior portion run out towards the point without impinging on the median keel.

This spine does not appear to be in close relationship with any of Ctenacanths hitherto described. Its short wedge-shaped form in the part exposed, and the extremely large and strong base, serve to distinguish it from all other species of the genus. I suggest the specific name minor as signifying its relatively small size.

III. ON THE OCCURRENCE OF THE KEUPER BASEMENT BEDS IN THE NEIGHBOURHOOD OF NOTTINGHAM.

By E. WILSON, F.G.S., and J. SHIPMAN.

ITHERTO, exposures of the junction of the Bunter formation brown sandstones and red marls of the "Waterstones," with a band of dolomitic conglomerate at the base, resting directly on an eroded. surface of the Bunter Pebble Beds. Recently, however, between

1 J. Shipman, "Conglomerate at the Base of the Lower Keuper," GEOL. MAG. 1877, p. 497.