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whole of the crystalline series above noticed is of volcanic origin, and consists of the lava and ashes of volcanoes, which have since been altered into these crystalline schists. A like view of the origin of the similar Huronian rocks of Lake Superior was some years since put forth by Nicholson, and G. M. Dawson has also expressed the opinion that a series of rocks in British Columbia, apparently lithologically identical with these, is of Mesozoic age and of volcanic origin. There is nothing, however, in the chemical or lithological character of these rocks in central or in eastern America to support such an hypothesis, nor any good reason for believing in the possible transformation of lavas and volcanic ashes into such crystalline schists. These rocks, at least in the regions first mentioned, and in Europe, are Eozoic strata; and the various hypotheses of which, under the name of pietre verdi, they have been the subject in Italy for the last half century, instructive in this connection (Record for 1876, page c). From these ancient crystalline rocks, we pass, by a natural transition, to the
PRE-CAMBRIAN ROCKS OF WALES. Rising from below the Lower Cambrian (Harlech) strata at St. David's, in South Wales, is a narrow ridge of crystalline rocks, which was described by the geological survey of Great Britain as partly intrusive and partly altered Cam. brian strata. Later studies by Hicks and Harkness have, however, shown that these crystalline rocks are marked by bedding-planes, and belong to two unconformable stratified series, the upper of which contains pebbles of the lower, while the basal beds of the unconformably overlying Cambrian present a conglomerate containing portions of both of these older series, which were clearly crystalline rocks before the deposition of the Cambrian. To the lower crystalline series Hicks has given the name of Dimetian, from Dimetia, an ancient kingdom including this part of Wales; and to the upper that of Pebidian, from Pebidiauc, the Welsh name of the hundred or district. The Dimetian rocks, which have a northwest strike, are nearly vertical in attitude, and have an estimated thickness of 15,000 feet. They are described as quartzose and feldspathic schistose strata, often greenish and purplish in color, with unctuous surfaces, besides thin beds of impure dark-colored ferriferous limestone, holding a greenish mineral, designated as serpentine or chlorite. A section from one of the more silicious beds showed by microscopic examination quartz and orthoclase interwoven as in a graphic granite, together with a plagioclase feldspar and chlorite. The rocks of the Pebidian series, which, with a northeast strike, rest unconformably upon the Dimetian, are usually nearly vertical and sometimes inverted in attitude. The portions exposed from beneath the overlapping Cambrian strata show a thickness of about 3000 feet. Besides compact conglomerates, the materials of which are apparently derived from the ancient Dimetian series, the Pebidian rocks are somewhat vaguely described as consisting of stratified porcellanites, alternating with greenish and purplish schists. They are cut by dikes, which do not traverse the overlying Cambrian. Besides the locality at St. David's, Hicks has recognized several areas of Pebidian strata in that region, which have been heretofore mapped as altered Cambrian strata, and others which have been regarded as intrusive rocks.
ROCKS OF THE ARDENNES. The crystalline rocks of the Ardennes have been the subject of microscopic study by De la Vallée-Poussin and Renard, according to whom many rocks hitherto regarded as exotic or eruptive, including hornblendic, euritic, and porphyritic masses, are really indigenous rocks, interstratified with the schists and quartzites of the region. They infer that these rocks were of aqueous origin, and became crystalline soon after their deposition.
GEOLOGY OF VERMONT. J. D. Dana has called attention to the observations of the late Mr. Wing on the geology of Vermont. The facts are, however, for the most part, not new, and Mr. Wing's observations were examined and discussed by Billings and the present writer in the American Journal of Science in 1868. It has long been known that the folded strata along the western base of the Green Mountains included fossiliferous rocks of Trenton age, both in New York and Vermont, and in the province of Quebec. In the latter region, strata supposed by Logan to be at the base of the Quebec group, and to under
lie the whole Green Mountain series, are of Trenton age, and owe their apparent position to overturned folds, accompanied in some places, apparently, by dislocations. In the province of Quebec they are involved both with the older crystalline strata and the Upper Taconic, but farther south, in Vermont, with the Lower Taconic series, which is, for the most part, concealed between Lake Champlain and Quebec. The crystalline limestones of this latter series in western New England have, by different geologists within the last twenty years, been referred to the Quebec group, to the Trenton, and to the Niagara or Helderberg-a history recalling that of the similar limestones of Italy which is told in the last year's Record (page xcix). In each case both stratigraphical and paleontological evidences have been appealed to in support of the various hypotheses, as in the similar and long-contested problems relating to the rocks of the Maurienne and Tarentaise in the Alps. Fossiliferous strata included in folds, or in faults, are supposed to fix the age of the entire series; and the absence of fossils from the other parts of the section is accounted for by the assumed metamorphosis of these portions of the strata. In more cases than one in the history of these rocks, forms not of organic origin have done duty as fossil remains. It is instructive, in this connection, to note the observation of Dana, that some slaty quartzites interstratified with limestones in Vermont exhibited “forms that looked exceedingly like casts of a Pleurotomaria and a Murchisonia, and of a valve of Orthis lynx." These, however, he admits to be only “imitative forms,” due in some unexplained way to concretion. There are also, according to him, sections of long-chambered cylinders, as of“crinoidal stems, yet having the chambers too large and irregular for any known crinoidal forms." From supposed paleontological evidence, he now refers the whole belt of Lower Taconic rocks to the Champlain division, thus returning to the view long ago advocated by Mather, but rejected by Logan in favor of one, and by Adams of another, hypothesis-each of which has found its advocates in turn.
GEOLOGY OF WISCONSIN. Irving has discussed the subject (alluded to in the last year's Record) of the imagined Paleozoic age of the Huronian rocks of Lake Superior, and declares that the facts now established in that region show (1) the existence of an ancient gneissic and granitic system, regarded as Laurentian, which is overlaid unconformably by (2) a series of quartzites, schists, and diorites, with limestones and some gneiss and granites, referred to the Huronian. This is followed, probably unconformably, by (3) the copper-bearing series (Keweenian), which includes greenstones and melaphyres, and also great thicknesses of interstratified sandstones, shales, and amygdaloids, the whole having a thickness of several miles. These are finally covered unconformably by a series of unaltered horizontal sandstones, including numerous fossils related to those of the Potsdam sandstone. Irving observes: “As to any of the Wisconsin or Michigan crystalline rocks being altered equivalents of the Primordial and newer strata of the Eastern States, such an hypothesis is certainly untenable for a moment.” And, while admitting that such things may be elsewhere, adds: “There certainly has been no period of metamorphism in the region of the Northwestern States since the beginning of the Primordial.” Mr. Sweet, who has particularly studied the rocks of this region, finds the thickness of the Huronian, so far as observed, about 5000 feet. The Keweenian, which extends in the form of a great synclinal from Lake Superior westward across the whole State of Wisconsin into Minnesota (a length of over 300 miles, and from thirty to fifty miles wide), has a thickness of strata estimated at not less than 40,000 or 50,000 feet. The trappean rocks of this series, as seen at the Falls of the St. Croix, were, by Owen, regarded as more recent than the Potsdam sandstone. It is, however, shown by Sweet that the trappean rocks in question not only belong to a stratified series, but that the basal beds of the sandstone resting upon them are in part made up of the ruins of these rocks. These sandstones have yielded several species of Conocephalites, Agnostus, and Dikellocephalus, besides Obolella and Lingulepis.
EOZOIC ROCKS WEST OF THE MISSOURI. Hunt has published some preliminary observations on the crystalline rocks of certain parts of the Rocky Mountains. The gneisses in the Colorado range as seen in Clear Creek cañon, and in the Sangre de Cristo range, near Garland,
have all the lithological characters of the Laurentian as displayed in the Laurentides, the Adirondacks, and in the South Mountain between the Hudson and Schuylkill rivers. They consist of gneisses frequently granitoid, often hornblendic, but scarcely micaceous, which are penetrated in the vicinity of Georgetown, Colorado, by well-marked granitic massesprobably exotic. Similar gneissic rocks are found in Glen Eyrie and the Ute Pass, in Colorado, where large masses are often highly granitic in aspect, with rarely interbedded gneissic layers. These rocks, which the writer regards, with Marvine, as indigenous, had already been by the latter observer compared with the Laurentian. The red granitoid rocks at and near Sherman are also by Hunt regarded as Laurentian gneisses. Labradorite rocks having the characters of the Norian series, and associated, like that series in the east, with large masses of titanoferrite, are known in Wyoming Territory.
The gneissic rocks of the Wahsatch range, as seen in the Devil's Gate on the Weber River, are also Laurentian, to which are referred the similar stratified rocks found in the same range farther south, in the upper part of the Little Cottonwood cañon. Here, among loose blocks of the gneiss, are found occasional masses of coarsely crystalline limestone with mica, and varieties of pyroxenic rocks characteristic of the Laurentian. In the lower part of the same cañon are, however, well-marked eruptive granites. The crystalline schists met with at the western base of the Sierras in Amador, Placer, and Nevada counties, in California, are described as having all the characters of the Huronian series as seen in Eastern North America and in the Alps. To this horizon, also, Hunt refers the similar crystalline rocks of the Coast range of California, as seen near San Francisco and near San José. The auriferous quartz veins in the counties above named are found traversing alike the Huronian schists and the granites of the region, which are, probably, newer than the schists.
EOZOIC ROCKS OF THE BLUE RIDGE. Referring to the observations on the geology of North Carolina in the Record for 1875 (page c), we may notice that Hunt has given a preliminary account of his late observations in a section across the Blue Ridge through Mitchell