IV. LOCAL METAMORPHISM.

In the second part of this paper I have asserted that the silicated minerals of crystalline rocks have a two-fold origin. In the first place, they may result from the molecular change of silicated sediments. These are either derived from the mechanical disintegration and partial decomposition of pre-existing silicates, or have been generated by chemical processes in waters at the earth's surface. In this way, steatite, serpentine, pyroxene, hornblende, chlorite, and in many cases garnet, epidote, and other silicates are formed by a crystallization and molecular re-arrangement of chemically-formed silicates, in a manner analogous to that in which mechanically-derived clays are converted into crystalline species. I have however pointed out that in the second place many of these silicated minerals may be generated by chemical reactions which take place among the mechanically mixed elements of sediments under the influence of heat aided by alka line solutions. Both of these methods are involved in rockmetamorphism, and in the case of the local alteration of rocks by igneous masses, it is easy by comparative examinations to trace the chemical changes involved in the production of silicated minerals by the second method. In this way, Delesse has shown that in several cases, where the chalk of Ireland has been altered by the proximity of intrusive traps, the sand and clay which the former contains have been converted into calcareous silicates. (Ann. des Mines, [5], xii, pp. 189, 208, 212.)

An instructive example of this process is furnished at Montreal, where the bluish fossiliferous limestone of the Trenton group is traversed by dikes of dolerite, which are subordinate to the great intrusive mass of Mount Royal. The limestone for a distance of a foot or two is hardened, but retains its bluish tint. Within a few inches, it is changed to a greenish-white color, which is seen to be due to a granular mineral dis seminated in the white carbonate of lime. The unaltered lime. stones from the vicinity contain variable amounts of insoluble argillaceous matters. A specimen treated with dilute chlorhydric acid, left a residue of about twelve per cent of a fine clayey substance, colored by a small amount of carbonaceous matter, and mixed with a little pyrites, which was removed by dilute nitric acid. This residue, after ignition, gave to a solution of carbonate of soda, 9.5 per cent of its weight of soluble silica; and the insoluble portion, being submitted to analysis, gave the result I. A portion of the limestone which was near to the intrusive rock, and was hardened and partially altered, was subjected to the action of dilute nitric acid, and gave an insoluble residue with the composition II. The more thoroughly altered greenish limestone was also treated with dilute nitric acid, which

dissolved the carbonate of lime, and left a residue, the analyses of which, from two different portions of the rock, are given under III, and IV.

The residue from the unaltered limestone, including the silica soluble in alkalies, contains nearly 75.5 hundredths of silica, and 16.5 of alumina. These, in the vicinity of the dolerite, have become saturated with protoxyd bases, including the small portions of magnesia and of oxyd of iron which the limestone contains. This process evidently involves a decomposition of the carbonate of lime, and the expulsion of the carbonic acid. is worthy of remark that while the unaltered limestone contains a little carbonate of magnesia, the rock from which III. was obtained yielded not a trace of magnesia to dilute nitric acid; II. marks an intermediate stage in the process, and shows moreover that the alkalies are still retained in combination with the aluminous silicate. These amorphous silicates, which have been formed by local metamorphism, might, under favorable circumstances, have crystallized in the forms of feldspar, scapolite, garnet, pyroxene, or some other of the silicious minerals which so often occur in metamorphic limestones. The agent in producing these silicates of protoxyds at the expense of the carbonates of the limestone, was probably a portion of alkaline salt, either derived from the feldspathic matter of the limestone, or possibly infiltrated from the contiguous feldspathic rock; whose elevated temperature produced the reaction which has resulted in thus altering this limestone.

Similar examples of local alteration are met with in several other places near to the intrusive rocks of the Montreal group. The schists of the Utica formation in contact with a dike of intrusive rock at Point St. Charles, and also near a mass of trachyte on a small island opposite the city of Montreal, occasionally exhibit small crystals of pyroxene, and in some cases prisms of hornblende. Among similarly altered shales at Rougemont are beds which consist of a highly ferriferous crystalline dolomite intermingled with dark-green cleavable hornblende, which forms thin layers, or in other cases encloses small rounded masses of the dolomite. (See for a description and analyses of this rock, the Geology of Canada, p. 634.)

At Montarville, the shales of the Hudson river formation are altered in the vicinity of the dolerite which forms the mass of the mountain. Some portions of the strata are very fine-grained, reddish-brown, and have an earthy sub-conchoidal fracture, with occasional cleavage joints. The hardness of this rock is not great, and it is apparently a kind of argillite, but between two beds of it is one of a harder coarse-grained rock, greenish-gray in color, and mottled with a lighter hue. This appears to be feldspathic in composition, and is penetrated in various direc tions by numerous slender prisms of black cleavable pyroxene, sometimes half an inch in length. The layers of sedimentation are distinctly marked in this bed, as well as in the finer grained strata which enclose it; and the whole affords an interesting example of the different effects of the same agency upon beds of unlike composition; although it would be impossible without comparative chemical analyses to determine whether the silicate which has here crystallized in the form of pyroxene, existed in the unaltered sediment, or whether, as in the case of the uncrystallized silicate from the altered limestone at Montreal, it had been generated under the influence of the intrusive rock. In by far the greater number of cases, the only apparent effect of the igneous rocks of the region under description upon the paleozoic limestones and shales, has been a very local induration; the appearance of crystals in these circumstances is a comparatively rare occurrence, and seems to depend upon conditions which are exceptional, showing, as I have elsewhere remarked, that heat and moisture are not the only condition of metamorphism. (This Journal, [2], xxxvi, 219.)

With these few examples of local metamorphism I conclude the present paper, proposing however to give in a subsequent one the results of some investigations of certain indigenous crystalline rocks.

Montreal, March 15, 1864.

ART. XVI.-Description of a new species of Chiton; by WILLIAM PRESCOTT, M.D., of Concord, N. H.

CHITON CALIFORNICUS.-The specimen of Chiton here described is in a dried state, having several years since been found cast upon the seashore at Santa Cruz Bay, California. It must, therefore, have been considerably larger when found than it now is. The color has also undergone a material change. It is said, when found, to have been of a uniform brilliant red: but now, the most prominent portion of the dorsal surface, (which would

AM. JOUR. SCI.-SECOND SERIES, VOL. XXXVIII, No. 113.-SEPT., 1861

naturally be the first to dry,) is mainly of a chesnut red, while the sides, together with some irregular blotches on the back, are of a dark brown, approaching in many places to black-occasioned, no doubt, by incipient putrefaction previous to becoming dry.

The whole length of the specimen is 6-8 inches; greatest breadth, 3.1 inches; greatest perpendicular height, 17 inches. The coat of mail, or shell-like covering, which gives shape and form to the whole animal, is ovate-oblong, convex above, considerably narrowed before and much wider posteriorly. It consists of eight testaceous pieces, or valves, which are imbricate (over

1.

Outline of 4th and 5th valves of C. Californicus, natural size.

lapping each other), with the extremities of their anterior wings deeply imbedded and concealed beneath the skin or mantle. Valves smooth, destitute either of lines of growth or geometrical markings and not carinated. A convex tubercle or prominence occupies the centre of the dorsal portion of each, being most prominent on the 2nd, 3rd, 7th and 8th, much less so on the 4th and 5th, while on the 1st and 6th it is in the form of an elongated ridge on the former longitudinally, on the latter diagonally. (The latter may be accidental, or the result of an injury.) On the 7th and 8th valves the form is more perfectly mastoidal and more distinctly pointed.

The first, or anterior valve, is externally in the form of an obtuse V, with four deep narrow fissures at its anterior inner margin. The posterior or 8th valve is rounded externally, with a small emargination or circular notch at its posterior margin.

The shape of the six other valves is papilioniform, or butterfly-shaped, neither keeled nor beaked; posterior margin lunated, being deeply and circularly arched between two wing-like projections which extend backward, their rounded extremities overlapping the valve immediately behind. The anterior wings, which are much larger than the posterior, are rounded, and project forward beneath the valve immediately before it, forming an arch much deeper than that formed by the posterior wings, but not so regularly circular.

The whole exterior surface of the mantle, from the margin upward, is thickly besprinkled with minute granulations which are of the same color as the mantle, most numerous on the sides and resembling shagreen, less numerous on the back, and least of all on the convex tubercles. There are also numerous indentations, which are most numerous on the convex portion or back, giving it the appearance of having the granules removed by friction.

The margin is narrow, and, with the whole inferior surface, is coriaceous.

The gigantic size of the specimen and the peculiar and unique form and structure of its valves render it extremely interesting, and a full description and figure of it very desirable and highly important.

The name of gigas at once suggested itself; but as that name had already been given to an African species (although one much smaller than this), the name of Californicus (from the State upon whose shores it was found) has been adopted.

It was obtained in California, some 8 or 10 years since, by the Rev. O. C. Baker, of this city, in whose possession it still remains.

Concord, N. H., March 20, 1863.

ART. XVII. On the rising of Springs and Streams in California, before the winter rains; by H. GIBBONS, M.D., of San Fran

cisco.

IT is a subject of popular remark in this country, that the springs and small streams begin to rise a long time before the setting in of the rainy season, and before a drop of rain has fallen. The common notion is that the rise of the springs has some relation to the near approach of the rainy season. Although I had no doubt of the fact for several years, yet the demonstration of it did not occur to me until the autumn of 1858.