slate, it yields readily to the knife, and is rather friable. The shale emits a bituminous odour when heated, which is not the case with the clay slate. From these characters and others. which are to be found in mineralogical works, it is evident that these substances differ greatly from each other; and we may soon learn to distinguish them. Nothing like the shale has yet been seen among the rocks of Winter hill.

In various places on this hill beds of greenstone are seen crossing the roads, and interstratified with the clay slate. We also notice many large detached rocks which are foreign to the strata, as the boulders of coarse granite, gneiss, mica slate, and sienite; these are probably of diluvial origin.

The slate is in most parts of the hill, where visible, crossed by innumerable seams running in all directions, and the surfaces of the angular fragments into which it is divided are covered with a ferruginous crust, arising from the chemical change that has taken place in the iron pyrites with which the rock is more or less charged.

In a direction nearly parallel with Winter hill, and but a short distance from it on the southwest, is Prospect hill. This hill is about 120 feet in height, and exhibits, like most of the eminences in this part of the country, that gentle acclivity, and rounded summit, so common in the transition formations of the Wernerian school. The lowest rock which presents itself on the southeastern side of this hill is a greenish compact felspar, strongly resembling in some parts of the bed, some varieties of limestone. This is an extensive bed, and may be traced in various parts of the hill in a direction from the southeast to the northwest. It has an imperfect slaty structure, and passes into clay slate. It is divided into somewhat irregular layers, and dips to the south, inclining a little to the west, under an angle varying from 20° to 50°. Towards its northwestern extremity it is seen having more of the character of clay slate, and covered by a mass of trap divided into imperfect prisms. This superincumbent trap is probably the remnant of an extensive bed which once covered the whole of the compact felspar, and this opinion is in some measure supported by the appearances observable at the junction of the two rocks. The subjacent bed, as already remarked, has many of the characters of clay slate, but in the immediate vicinity of the trap rock has a degree of hardness, a compact structure, and fracture almost like that of hornstone, the slate seems to have undergone a great and remarkable change. The same appearances may be noticed

in various parts of the present surface of this bed. It is also probable that the trap rocks seen near the summit of Winter hill were once a part of the same mass, which may have been connected with the greenstone now seen between the two extremities of the bed of compact felspar. Thus we should have the latter surrounded and covered by trap rocks, which in whatever manner it may be explained, are now known to have affected the texture of the rocks with which they came in contact.

The mass of trap which I have remarked as interposed between the two extremities of the bed just described, has more of the characters of sienite than have any other of the trap rocks of these hills. It is distinctly seen on the southwestern side of Prospect hill, where it has been quarried for architectural purposes. The general characters of this bed are those of distinct greenstone, the hornblende and felspar being present in it in nearly equal proportion: but in some parts it contains quartz and is distinct sienite. This rock is one of those which we so often find having characters belonging to two distinct compounds, and to this the term sienitic greenstone is peculiarly applicable. In some parts of the bed the felspar predominates, and has a fine flesh colour; in one place the prisms of felspar are an inch or more in length, and cross each other in all directions, leaving angular spaces, which contain decomposing prehnite, and, rarely, distinct crystals of quartz. These prisms of felspar are somewhat peculiar, as they uniformly have an exterior portion of a dark brown colour, investing the mass of the crystal which is of a lively flesh tint: when broken longitudinally, the contrast is striking.

This sienitic greenstone is crossed in various directions by fissures, the walls of some of which are encrusted with thin layers of felspar; others are filled up with this mineral. It was in one of these veins of felspar that prehnite was formerly met with, but of late no good specimens have been found; I have noticed minute portions of it however in other parts of the bed. The occurrence of the same minerals in the rocks of this country, which we have seen in those of Europe, either by personal examination of their localities, or by the interchange of specimens, is no small addition to the interest of our researches in the almost untrodden field which the geology of our country presents to us. The prehnite of Charlestown the most experienced eye would be unable to distinguish from that of France, Tuscany, or Scotland. This mineral is so commonly associated with rocks of the trap family that it may almost be viewed as characteristic of them.

tribute the formation and present situation of trap rocks to the operation of intense heat, would probably be inclined to seize upon this spot as exhibiting many circumstances in support of their favourite theory ;-while those who maintain the opposite opinion would with great plausibility attempt to explain the present appearance of the strata, as a confused crystallization from a disturbed solution, and would cite numerous analogous examples. It is obvious that it is difficult, if not wholly impossible, in the present state of geological science, to give a satifactory explanation of this and many similar appearances. In regard to the calcareous cement, there can be little doubt that it is of comparatively recent origin, and has been deposited in the fissures subsequently to the great disturbance and dislocation of the strata of slate. The facts even in this limited space, are, however, sufficient to prove a succession of epochas, and the observer, uninfluenced by systems, will see all the unequivocal vestiges of a series of strata originally horizontal, but which have been broken and disturbed by subsequent changes; the effect of revolutions, of the antiquity and extent of which we are as ignorant, as of the agencies by which they were produced.

I shall now proceed to offer some remarks on a series of rocks situated to the southwest of the peninsula of Boston.

These rocks are seen in the towns of Roxbury, Dorches ter, Brookline, Brighton, Newton, Needham, Milton, Dedham, and Quincy. The principal rock of this series is a coarse conglomerate which has by several writers, in this country, been called Grau Wacké, and by some Wacké. In the years 1807-8, the attention of several gentlemen of Boston was first awakened to geological inquiries, during the short residence in this place of an amiable and accomplished French mineralogist, Mr S. Godon, to whom we are indebted for the first course of lectures on this science ever given in Boston. Mr Godon communicated to the American Academy of Arts and Sciences some remarks on the rocks and minerals of the vicinity. This gentleman was unfortunately deeply infected with that love of minute distinction among the subjects of his inquiries, and that proneness to the coinage of new names which characterizes the geological writings of the French school, and we find in his memoir the same rock divided and subdivided into five or six varieties, to each of which a name is given that we in vain look for in the standard works on the science. The name which Mr Godon applied to the rock we are considering, is one which no other writer before him had

given to rocks of this character, and there can be little doubt that when he speaks of this rock as "Wacke" he intended Grau Wacke; it is not improbable that an accidental omission occurred in printing the memoir.

It has however happened that this omission has misled others, and the error in Mr Godon's publication has been copied by succeeding writers, and we hear it daily repeated that Wacké occurs abundantly in the vicinity of Boston.

These remarks may seem unnecessary, but in this country, where so many young and ardent minds are yearly commencing the study of Geology, we cannot be too much on our guard against erroneous impressions at the outset. Not however resting on mere assertion, let us endeavour to point out the distinction between these very different rocks, from the writings of those who have enjoyed the best opportunities for observation, and whose opinions are to be received with respect and confidence.

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Prof. Jameson describes Wacké as follows: "its colour is greenish grey, of various degrees of intensity"-" those varieties that incline to basalt, approach in colour to greyish black." "The fracture is large and flat conchoidal in the most characteristic specimens; but small grained uneven in those which are less so."*" It is more or less shining in the streak. It is soft. It is sectile. It feels rather greasy.' 3966 Before the blowpipe it melts into a greenish slag." (System of Min. 1816.) The description given by Mr Phillips differs little from the foregoing. "The Wacké of Werner," says M. Brochant," is a substance intermediate between basalt and clay. It contains less iron, and perhaps more lime and magnesia, than basalt; its texture is less compact, and it decomposes more readily." It is unnecessary to quote other descriptions of this substance, which would be little else than copies of those already given. How utterly inapplicable to the rock in this vicinity, named in Mr Godon's memoir, Wacké, are these descriptions, must be evident to the most superficial observer.

The name Grau Wacké was given by the German miners to a peculiar rock composing the greater part of the mountains in which their labors were carried on; it was adopted by Werner, and has been received among geologists of all countries, although liable to serious objections, as has been

Highly characteristic specimens of the Wacké described by Prof. Jameson, in the 1st vol. of the Edin. Jour. page 138, may be seen in the Minera logical cabinet at Cambridge.

remarked by Dr Macculloch, not only by reason of the cacophony of the term, but also as it does not express any distinguishing character of the rock. The confusion which has arisen from the use of this term is a serious evil in geology, as it has been applied to a great variety of rocks essentially different from each other, and from the genuine grey wacké of Werner. The definition of Werner is precise, and he applied the term only to such rocks as were composed of mechanically altered portions or fragments of quartz, indurated clay slate, and flinty slate, cemented by a basis of clay slate. It has been attempted of late to extend this definition to all granular and fragmentary rocks, still, however, considering the cementing substance and mechanical structure as the essential part of the definition.

Grau Wacké having been placed by Werner at the head of his transition rocks, has become a convenient term under which those, who have deemed it essential to their reputation as geologists to find transition rocks succeeding the primitive in the districts they have attempted to describe, have placed all those which they have not been able to recognise as members of the latter class. They have described what they conceived ought to be found, not what actually occurred, and from this blind reliance on a favorite hypothesis, have given us breccias of all sorts, sand-stones, clay-slates, and various other dissimilar rocks, as Grau Wacké.

Grau Wacke, says Prof. Jameson, "is composed of angular or other shaped portions of quartz, felspar, Lydian stone, and clay slate, connected together by means of a basis or ground of the nature of clay slate, which is often highly impregnated with silica, thus giving to the mass a considerable degree of hardness. The imbedded portions vary in size, but seldom exceed a few inches in breadth and thickness. When the imbedded portions become very small, the rock assumes a slaty structure, and forms the grey wacké slate of geognosts. When the grains almost entirely disappear, and the rock is principally composed of clay-slate, it is named transition clay-slate."

M. Brochant, who gives nearly the same description, does not include under it those varieties in which the grains (imbedded portions) exceed the size of a hazel nut. The Traumate of Daubuisson and some of the varieties of Psamite of Brongniart, are grey wacké.

Grau Wacké, says Kirwan, "is a particular kind of sandstone, containing not only grains of quartz, siliceous schistus,