wider regions of absorption is continued till these intermediate regions of transmission disappear, the whole, to repeat an expression I have used in an earlier memoir, seems to become one continuous cold band, in which, however, we have found a little heat struggling through in the part beyond 11". Briefly, then, we may say, that to an eye which could see the whole spectrum, visible and invisible, the luminous part being, as we know, interrupted by occasional dark lines, the lower part to 5" would appear to consist of alternate bright and dark bands, and the part below 5" be nearly dark, but with feeble "bright bands at intervals. In conclusion, we may say that these new researches extend the known solar spectrum from three to much over the eighteen microns, shown on our plate, and while confirming the previously announced fact that the solar heat of the great wave-lengths which actually reach us is negligible in amount, show from the fact of the existence of any at all, that the anomaly of our being able to perceive lunar heat where we could not formerly detect solar, can be explained consistently with the possible existence of the latter of every wave-length before absorption.

These investigations into a problem of Solar Physics have also incidentally led us to the prospective means of solution of an important one in Meteorology, for they have opened to observation the hitherto unknown region of the spectrum, in which the nocturnal and diurnal radiations not only from the moon toward the earth, but from the soil of the earth toward space, are to be found and may be hereafter studied in detail.

ART. XLII.—A brief history of Taconic ideas; by JAMES D. DANA.

As the Taconic controversy is now nearing its end, a brief review of the progress of ideas relating to the various sides of the subject from the time of the earliest discussions to the present will be found instructive.*

* A historical account of the Taconic, especially of the views of Taconic advocates, entitled "The Taconic System and its position in Stratigraphical Geology,' by Mr. Jules Marcou, is contained in the Proceedings of the American Academy of Sciences, vol. xii, 1884, 1885, pages 174 to 256. It relates mainly to the 'Upper" Taconic, and makes little mention of facts or observations of the thirty years past connected with the "Lower" Taconic rocks in the southern half of Vermont, Massachusetts with its typical Williamstown Taconic section, and in eastern New York from Rensselaer Co. southward; all which have a profound bearing on the question of age, and on the true relation of the "Upper" and "Lower" sections. No account is given of the Taconic system of 1842, in the detailed presentation of which Prof. Emmons states its fundamental and distinctive

1818 to 1828.-The name Taconic first came into topographical geology through Professor Chester Dewey, who made the earliest geological map of the region, observed the general arrangement and eastward pitch of the beds, and called the rocks in the style of the time, Transition rocks.*

Prof. Amos Eaton gave credit in 1820 to Prof. Dewey for the chief part of his knowledge of the Taconic rocks; but, by 1828, this pioneer in American geology had made fifteen sections of the rocks from the Hudson River to the Taconic mountains, in order to ascertain, as he says, their conformability and true order of succession.

1836 to the close of 1841.-Prof. Emmons, whose department in the New York Geological Survey covered the northern and northeastern part of the State, and who was already acquainted with the adjoining region of Williamstown in Massachusetts, continued his survey into Williamstown. But in the New York Annual Reports no mention is made of the Taconic system either by him or by any other of the New York geologists. This is true even of the latest-the fifth-annual Report, that for 1840. Prof. Emmons, in his part of this Report, under the date of transmittal of Feb. 1, 1841, says on page 94:

"The Granular quartz of Bennington, which occurs also in Dutchess Co., N. Y., I believe to be Potsdam sandstone in a metamorphic form, and the granular limestone associated, to belong to the same geological epoch." I believe "that the rocks from Lake Champlain, along the eastern part of the counties of Washington, Rensselaer, Columbia, Dutchess, all of Putnam, Westchester, great part of Rockland and southeast part of Orange are metamorphic and intruded rocks, and I would suggest that all the rocks from the New York state line to the Connecticut valley are similar. The talcose and micaceous and talco-micaceous divisions of the Green Mountain range in Vermont, Massachusetts and Connecticut have a strong analogy to the metamorphic slates of the east part of Washington, Rensselaer and Columbia Counties, but are traversed by large granite veins and are interstratified with intrusive rocks which might be expected to produce a greater change in mineral constitution.'

characteristics, and to which alone the discussions in 1842 to 1844 by Rogers, Mather, Hall and Hitchcock had reference. Mr. Marcou's own special Taconic investigations were made in northern Vermont and its vicinity, a region barely alluded to as Upper Taconic by Prof. Emmons.

* C. Dewey, this J., i, 337, 1819; ii, 246, 1820; viii, 1, 240, with the map, 1824. †A. Eaton, Index to the Geology of the United States, 1820.

A. Eaton, this J., xiv, 147, 1828, where he says: "I have traversed the Transition range from Massachusetts line to Hudson River in fifteen places, since the first part of my survey was published, for the purpose of ascertaining the true superposition of rocks in this most complicated and difficult geological theatre." Prof. G. H. Cook recently drew my attention to this statement of Prof. Eaton which I had previously overlooked.

As late, then, as February, 1841, there was no indication that the Taconic system had taken shape in the mind of Prof. Emmons, although pretty well stored with Taconic facts. In opposition to the ideas of the Taconic system of 1842, the quartzyte is spoken of as the "Potsdam sandstone" in a metamorphic state, and the associated granular limestone as of "the same geological epoch."

In 1842, the quarto Report on New York Geology by Prof. Emmons appeared, with its letter of transmittal dated January 1, 1842; and in this volume the account of the Taconic system as a new system of rocks in American geology covers pages. 135 to 164, thirty quarto pages. The germinant period was, consequently, between February 1, 1841, and January 1, 1842.

Another fact brings us closer to the time of first announcement. The system became a subject of special discussion, as I am told by Prof. James Hall, at the meeting of the American Association of Geologists and Naturalists held early in April, 1841, at Philadelphia. This narrows down the germinating period to the two months between February 1 and April 10 of 1841.

Those who took a prominent part in this first discussion were Professors Henry D. Rogers, Edward Hitchcock, Wm. W. Mather, Mr. James Hall and Mr. Lardner Vanuxem. Prof. Rogers was interested in the subject because of his elaborate geological study of the Appalachian Mountain system on which he reported at length to the Association the following year; Prof. Hitchcock, because the heart of the Taconic region was within his own field of study-the state of Massachusetts, his final report on which he already had in the press; Prof. Mather, because part of the Taconic rocks west of Massachusetts were very largely within his section of the survey of the State of New York; Prof. Hall, because of the bearing of the facts on the system of New York rocks. Prof. Vanuxem, also one of the New York State geologists, had under his charge only southern New York west of the Taconic area and had given the rocks no study.

Although we have no report of the discussions, we learn from later publications that Professors Hitchcock, Rogers,. Hall and Mather objected to the views of Prof. Emmons, and Mr. Vanuxem+ favored them.

* The Proceedings of the meeting in this Journal, xli, 158, 1841, and in the Trans. Assoc. Am. Geol. and Nat., 1840-42, contain no allusion to the discussion, discussions being, as the Preface of the Transactions states, "imperfectly reported in these pages." The only fact of any Taconic interest mentioned in the Proceedings of the meeting for 1841 (Proc. Assoc., 1883, p. 16, and this J., xli, 163), is that of the occurrence of impressions of Annelids in the slate of Waterville, Me., by Prof. O. P. Hubbard—the fossils and slates being afterward (1844) claimed for the Taconic by Emmons. Prof. Wm. B. Rogers did not become a member until 1842. † Mr. Vanuxem has a page on the subject in his N. Y. Geological Report, p. 22.

The comparison of views at the meeting resulted in inducing Prof. Rogers and Prof. Hall to take the field for the study of sections over the Taconic region. The season had just passed when Prof Rogers made a report on his results to the American Philosophical Society at Philadelphia,* sustaining the views which Hitchcock, Hall, Mather and himself had before favored, namely: that the rocks were Lower Silurian (as the term was then used) from the Potsdam upward, but much flexed and disguised by partial metamorphism. Prof. Emmons mentions in his Report of 1842, on p. 147, that "Professors Hitchcock and Rogers" were prominent objectors to his views. We learn that Prof. Mather's views were essentially those of Prof. Rogers from his New York Geological report published in 1843. In his Preface he gives an interesting account of the discussion. The Dr. Dana he refers to was Dr. Samuel L. Dana, of Lowell, Mass.

•

1842 to the close of 1844; Phase I.-Such was the state of opinion when Prof. Emmons's full report on the Taconic system was published in 1842.

The Taconic system of this report comprised the rocks from the Hoosic Mountains westward, passing over the Hoosic Valley, Saddle Mountain (which included Greylock), and also over the high ridge of granular quartz, Oak Hill, just north, the Williamstown plain, the Taconic Mountains next west on the Massachusetts boundary, and the eastern border of New York west of this boundary to the Hudson. Beginning on the east, the rocks were: (1) the "Stockbridge limestone," (2) the "Granular quartz" and the "magnesian slate" of Greylock, which are topographically north and south of one another, (3) the limestone of Williamstown plain, (4) the "magnesian slate" of the Taconic mountain, (5) the "sparry limestone" west of the latter, and farther west, (6) the "Taconic slate." The following, commencing at the bottom, is the chronological order according to Emmons, and in a parallel column, are given the true equivalents (the Roman numerals giving the order of age) as established by the latest investigations.

Taconic System of 1842.

6. Stockbridge limestone.

4.

Equivalents.

II. Lower Silurian.

a. Magnesian slate of Grey- III. Hudson slate.

lock, perhaps a repeti

tion of No. 3.

b. Granular quartz.

Limestone.

I. Cambrian.

II. Lower Silurian.

3. Magnesian slate of Taconic III. Hudson Slate.

Mountain.

2. Sparry limestone.

1. Taconic slate.

II. Lower Silurian.

III, I. Hudson slate and Cambrian.

* Proc. Amer. Phil. Soc., Jan., 1842.

AM. JOUR. SCI.-THIRD SERIES, VOL. XXXVI, No. 216.-DEC., 1888.

26

The order of succession adopted by Emmons was the order of superposition, as he states, with arguments in its favor, on page 147 of his report, the idea of flexures being rejected. The granular quartz he says (p. 138) "lies between two masses of limestone," which is topographically a fact.

The idea of the pre-Potsdam age of the beds was based on the absence of fossils; on the difference in the kinds of rocks, and in their succession, from the lower rocks in the New York series; also on supposed unconformability, no special case however being mentioned. He says, with reference to the distinction in kinds of rocks (p. 139), "As a general rule, certain minerals are found in particular rocks; and may not a similar rule or law prevail where a system of rocks is concerned?"

Early in May of 1844, two years after the appearance of Prof. Emmons's Report and over half a year before his second presentation of the system, Professor H. D. Rogers brought the Taconic question into his Presidential address before the "Association of American Geologists and Naturalists," and reiterated his former conclusions.*

1844 to the close of 1849; Phase II-In December, 1844, appeared as a pamphlet in 4to-the preface bearing the date December 2, 1844-Prof. Emmons's revision of the Taconic system, with additions and an extension of its limits; and in 1846, the same was published as a chapter in his N. Y. Report on Agriculture. The prominent changes are the following: (1) the system is, for the most part, turned the other side up, Rogers's views being adopted as to flexures and overthrust folds. (2) It is made in part fossiliferous, and the fossiliferous part is put at the top under the idea that the fossils proved it to be the newer part. (3) The granular quartz iş placed at the bottom.

The order of the strata in the Report, and the equivalents, are as follows:

* H. D. Rogers, this Journal. xlvii, 137, 247, 1844. The address is a discussion of geological views, and is of permanent interest. His remarks on the Taconic system occupy pages 150 to 153; and he ends them with the suggestion that the Taconic system, instead of belonging exclusively to the Champlain division, may "include also some of the sandy and slaty strata here spoken of as lying beneath the Potsdam sandstone," referring to his own observations in Virginia and East Tennessee.