we are to be guided at all by palæontology we cannot regard this rock as lying at the top of the Lower Silurian but at the very base of Barrande's Second Fauna, if not indeed a little lower. It is therefore not the Medina sandstone, but a formation somewhere near the hori zon of the Potsdam. This accords exactly with conclusions drawn from the evidence afforded by the fossils discovered by our survey at Quebec last year."

In this paper the formation was first referred to the base of the Lower Silurian on the palaeontological evidence. The following notice from C. Hitchcock was published shortly afterwards. (See p. 454 of the vol. last cited):

"Letter from C. HITCHCOCK, Esq., on the first observation of the

Fossils of the Red Sandstone formation of Vermont.

"Eds. Silliman's Journal: As a notice of the Conocephalites from the Red Sandrock series in Highgate, Vt., has appeared in your Journal (Second Series, vol. xxxii, p. 232), it is but just to the dead to state who were the original discoverers of this trilobite. By referring to the Third Annual Rept. Geol. Vt., 1847, pages 14 and 31, it will appear that Prof. Z. Thompson conducted Prof. C. B. Adams to Highgate, where both gentlemen procured a large number of these trilobites. They were sent to Prof. J. Hall in 1847 for determination, who gave them the name Conocephalus, the same genus to which Mr. Billings now refers them. At that time the precise position of the Conocephalus was not known. Nor was Prof. Hall able to give more definite information respecting them in 1858 when I showed him the specimens again.

These trilobites are noticed on pages 339 and 340 of our Third Report on the Geology of Vermont, which will be ready shortly for distribution.

Amherst, Mass., Oct. 23d, 1861."

From the above I think it will be evident that I was the first to decide the age of the red sandrock on palæontological grounds. The locality at Highgate is perhaps not exactly of the age of the typical Potsdam, but nearly of that age.j

4.-Sir R. I. Murchison's Address.

In a paper entitled "On some points in American Geology," published in the Am. Jour. Sci. 2nd series, vol. xxxi, and in the Can. Nat. & Geol., vol. 6, 1861, Dr. Hunt gave an account of the determination of the age of the Quebec group, and introduced Prof. Hall's researches in such a manner that Sir R. I. Murchison was led to make the following statement in his "Address to

the Geological Section of the British Association, at Manchester, Sept. 1861"::

"In an able review of this subject, Mr. Sterry Hunt thus expresses himself:-"We regard the whole Quebec group, with its underlying primordial shales, as the greatly developed representatives of the Potsdam and Calciferous groups (with part of that of Chazy), and the true base of the Silurian system." "The Quebec group, with its underlying shales," this author adds (and he expresses the opinion of Sir W. Logan), " is no other than the Taconic system of Emmons;" which is thus, by these authors, as well as Mr. James Hall, shown to be the natural base of the Silurian rocks in America, as Barrande and De Verneuil have proved it to be on the continent of Europe."

The meaning of the above is simply this: that the age of the Quebec group was determined by SIR W. E. LOGAN, as physical geologist; PROF. HALL, as palæontologist, and by DR. T. S. HUNT, as chemist and mineralogist, an arrangement very satisfac. tory to the latter two gentlemen, but not so to myself. Upon reading the address, I resolved to publish same remarks upon it, but on speaking to Sir W. E. Logan, he thought it best that the matter should be rectified by himself. Accordingly he addressed the following letter to the Editors of the Amer. Jour. Science : "Letter from Sir WM. E. LOGAN, Director of the Canadian Geological Survey, on Sir Roderick Murchison's reference to the determination of the age of the Quebec Rocks.

Montreal, November 27, 1861.

"To the Editors of the American Journal of Science:

Dear Sirs,-In his address to the Geological Section of the last meeting of the British Association, Sir Roderick Murchison has placed the name of my friend Prof. Hall in such a relation to the Quebec group of rocks, as might lead to the inference that to him was due the credit of having determined its horizon, as adopted by the Geological Survey of Canada. Nothing I am persuaded can be farther from the mind of this distinguished palæontologist than a wish to put forward any claim of this description, as the credit is wholly due to Mr. Billings the Paleontologist of the Canadian Survey.

In 1848 and 1849, founding myself upon the apparent superposition in Eastern Canada of what we now call the Quebec group, I enunciated the opinion that the whole series belonged to the Hudson River group and its immediately succeeding formation; a Leptæna very like L. sericea, and an Orthis very like O. testudinaria, and taken by me to be these species being then the only fossils found in the Canadian rocks in question. This view supported Professor Hall in placing, as he had already done, the Olenus rocks of New York in the Hudson River

group, in accordance with Hisinger's list of Swedish rocks as given in his Lethæa Suecia in 1837, and not as he had previously given it. But the discovery in 1860 of the Point Lévis fossils enabled Mr. Billings to prove that the rocks of the Quebec group must be placed near the base of the Lower Silurian series instead of at its summit, and it thus became necessary to discover some other interpretation of the physical structure than the one suggested by the visible sequence of the strata.

Although there may be difficulties in regard to detail, the interpretation given in my letter to Mr. Barrande of the 31st December, 1860, will, I am persuaded, turn out to be the right one. Prof. Emmons long ago asserted that the rocks in question in Vermont were older than the Birdseye and Black River formation. In this I now agree with him; while however his interpretation of the structure would make them all older than the Potsdam sandstone, mine would not. But whatever the value of my present interpretation, it might have been some time before I should have been urged to look for it, had it not been for the paleontological skill which Mr. Billings brought to bear .on the question. I am, dear Sirs, very truly and respecfully yours, W. E. LOGAN.'

To the above I shall add two quotations from the last letter I received from Dr. Emmons on the subject of the Taconic system. He was, at the date of this letter, State Geologist of N. Carolina:

MR. E. BILLINGS:

"RALEIGH, Feb. 5, 1861.

"MY DEAR SIR,-I am much obliged to you for your favor of the 30th inst., and especially for the opinions and kind regards which you express. Be assured they are highly appreciated, and the more so seeing that they are rare. I had for years past looked upon the subject with a kind of indifference, until you had expressed to Col. Jewett, opinions favourable to the existence of the lower rocks I had contended for; not indeed that I had any misgivings of the truth of the position I had taken, for that would be impossible from all I had seen, provided there were truth in geology, and that the department were founded on principles. But the real difficulty has always been that geologists would not look at the question at all.”

"Be assured that I fully appreciate your kind aid in the matter of the Taconic system, for I think I should have gone down to the grave before it had been acknowledged, except for your active, intelligent, and disinterested labours in the cause."

"Yours truly,

"E. EMMONS."

FIG. 1. Dragram of the interior of the ventral valve of a specimen, supposed to be a small individual of O. Canadensis; bb, the two large sub-central muscular impressions; dd, the groove under the area; e, enlargement of the same; g, the pedicel groove in the area, on each side of which is a smaller oblique furrow; r, the ridge in front of the muscular impressions.

2. Interior of a dorsal valve; a, the area; c, the pair of small scars in front of the two larger. The other letters, the same as in Fig. 1. 3. Dorsal view of the original specimen.

4. Side view of the same.

5. Ventral view of the same.

A short notice of this genus was published in the last number of this journal, Dec., 1871. I now propose to extend the des cription, so far as our present material will admit.

(All the figures in this paper are of the natural size.)

Genus OBOLELLINA, Billings.

Generic characters.-Shell, unarticulated, ovate or orbicular, smooth or concentrically striated. Area of the ventral valve with a median groove, on each side of which there is, sometimes, an additional furrow. In the interior of this valve there are two large, ovate, or sub-rhomboidal muscular impressions. They are situated near the centre, but usually (for the greater part) in the posterior half of the shell. They are sometimes obliquely striated or grooved, or obscurely reticulated by both transverse and longitudinal striae. Close under the area there is a fine, but distinctly impressed groove, which curves outwards and forwards, outside of the muscular scars for a greater or less distance towards the front margin. There appears to be an enlargement of this groove, just under the peduncular groove of the area, on the median line, as if for the attachment of a muscle. The large scars are bordered anteriorly, by an elevated margin, which is prolonged forwards, along the median line, in a more or less prominent ridge; this ridge varies greatly in the amount of its developement, in different individuals of the same species, being sometimes almost obsolete.

The area of the dorsal valve varies greatly in size in the different species, and is either flat or with a triangular elevation under the beak like a pseudo-deltidium. Beneath the area there is a fine groove, which curves outwards and forwards, as in the ventral valve, with a similar enlargement in front of the beak. There are two large, ovate, sub-central muscular impressions, with a smaller pair in front of, or between them. These latter are situated on or close to the median line, and usually appear as a single scar, but in some specimens are distinctly divided into two, by a longitudinal ridge. Their form varies in different individuals of the same species. The muscular impressions are margined, anteriorly, by an elevated border, which is extended forwards as an obscure ridge, a greater or less distance towards the front.

In the original notice it is stated, that this genus has no cavi. ties in either valve. This holds good for all the specimens of O. Canadensis of which the interior has been seen. In O. Galtensis, however, while some of the specimens have no cavities, in others, as is shown by the casts of the interior, there is a small one extending a short distance under the larger muscular scar on each side in the ventral valve. In one of our specimens there is a short cone, half-a-line in length, on the edge of the cast of the cavity. No