[23] Manganese also occurs in the mineral youngite, which contains lead, zinc, iron, manganese and sulphur, but the mineral is considered of doubtful homogeneity. (See System of Mineralogy, E. S. Dana, 1892). [24] When manganese is precipitated artificially as sulphide it is usually in the form of the monosulphide (MnS), in either a hydrous or an anhydrous form. [25] Manganese occurs in various hydro-silicates, but they do not appear to be deposited as sedimentary strata in the same manner as glauconite. [26] If the water moved very slowly, the deposition would probably take place approximately in the same spot; if the waters moved more rapidly, the iron might be deposited in one place and the carbonate in another, in the way explained on page 363. [27] The hydrous oxides of iron are not crystalline. [28] See p. 363. [29] Bischof suggests that the action described by Fresenius causes the separate deposition of iron and manganese; and also that it explains the occurrence of large deposits of manganese ore in regions where the iron ore contains least of that ingredient. (See Elements of Chemistry and Phys. Geol., Vol. III., pp. 531–532.) [30] In some cases these iron and manganese deposits are undoubtedly formed by the replacement of limestone or other rocks, as is further discussed on pages -- to --. [31] Kentucky Geol. Survey, Report of Progress, Vol. III., New Series, 1877, p. 164. [32] U. S. Geol. Survey, Tenth Annual Report, 1888–1889, Vol. I., pp. 409–422. SOME RIVERS OF CONNECTICUT.[33] _Outline._--Introduction.--Topography of Connecticut: The upland plateau, its origin, date, elevation, valleys sunk beneath its surface.--Lowland on the Triassic area.--Later oscillations.-- Résumé of the topography.--Early drainage.--Re-adjusted streams. --Revived streams.--Unconformable rivers, consequent or superimposed.--Pleistocene changes; the Farmington, Quinnipiac, Scantic.--Abandoned gaps. _Introduction._ In order to study intelligently the history of a river, one must first become acquainted with the present physical geography of the region in which the river lies, and know the stages of its development. Therefore, before classifying the rivers of Connecticut, I shall consider the topography of the state, and in a few paragraphs outline the successive cycles in the history of its growth. The scope of this article will not permit a discussion or even a full statement of the evidence on which these conclusions are based. They have been stated at considerable length by Professor W. M. Davis,[34] and the reader is referred to his papers for the complete discussion. His conclusions in respect to the physical geography are accepted here without question, and form the basis for the discussion on the rivers of the state.