I forward you specimens of straw and rose-coloured amianthus I recently met with on Staten-Island, which I detached, in strips, from a rock; it not appearing, as is usual, in veins. It breaks up like flax, and may be spun and wove without the aid of moisture; and in respect to tenacity, flexibility, and length of fibre, it may be considered the best found in this country, and perhaps equal to any hitherto discovered. Staten-Island exhibits many minerals worthy of examination. I subjoin, as requested, the following geological description, &c. Hoboken, where I discovered native carbonate of magnesia, is situated opposite the city of New-York, on the western or New-Jersey bank of the Hudson. It is a primitive, insulated elevation, with a nucleus of serpentine; the ground gradually descends in every direction except on the river side, where mural precipices of serpentine rock are observed, extending about 100 rods parallel with the water, and elevated from 60 to 100 feet above its level. The carbonate of magnesia I found in horizontal veins of nearly two inches in breadth, and of unknown depth, in a midway region of this serpentine ledge; I extracted a considerable quantity with a spoon. When first taken out it was soft, white, and very slightly adhesive, from a little moisture; but, when dry, fell to powder without friction. The nature of the mineral I immediately conjectured, and treated it with diluted sulphuric acid, in which it entirely dissolved with effervescence, forming a bitter fluid, and leaving no sediment. Upon evaporation, well-defined crystals of Epsom salts were formed. It differs little from the manufactured carbonate of magnesia of the shops; but is rather a super than a sub-carbonate. It has been analyzed by Professor Mitchill, who found it exclusively composed of magnesia and carbonic acid. Carbonates of magnesia, hitherto discovered, have been, I believe, found impure, and in a state of rock, requiring chemical process to render them serviceable; this is, perhaps, fit for immediate use. When I first mentioned the discovery to mineralogists, they were incredulous, supposing it did not natively exist in this state, but I convinced them by uniting it with sulphuric acid. REMARKS. The specimen of amianthus, referred to in Mr. Pierce's communication, is uncommon. The fibres measure from 12 to 15 inches in length, and are as soft and flexible as fine human hair. It will be remembered, that in the rocks at Hoboken, Dr. Bruce discovered the hydrate of magnesia, or magnesia combined with nothing but water, in the proportion of about 70 per cent. of magnesia. This discovery gave a new and interesting species to mineralogy; it is now admitted in the systematical works on mineralogy.