My attention was directed to the plaster stated to exist on the St. Martin Islands. These islands compose a small group lying about nine or ten miles north-northeast of Michilimackinac. Captain Knapp, of the revenue service, had been requested to take me to the spot with the revenue cutter under his command. I was accompanied by Captain Douglass, of the expedition, and by Lieutenant John Pierce, U. S. A., stationed at the fort. The gypsum exists in a moist soil, not greatly elevated, during certain winds above the lake. Pits had been dug by persons visiting the locality for commercial purposes. It occurs in granular lumps of a gray color, as also in foliated and fibrous masses, white, gray, chestnut color, or sometimes red. No difficulty was encountered in procuring as many specimens as were required. This group of islands is noticeable, also, for the large boulder masses of hornblende and granite rock, which are found imbedded in, or lying on the surface, along with fragments of breccia, quartz, &c. This drift is more abundant, on all the islands I have seen, as we approach the north shores of Lake Huron. Having completed the examination of these islands, we returned to the harbor after an agreeable excursion. To observe the structure and character of the Island of Michilimackinac, I determined to walk entirely around it, following the beach at the foot of the cliffs. This, although a difficult task, from brush and debris, became a practicable one, except on the north and northwest borders, where there was, for limited spaces, no margin of debris, at which points it became necessary to wade in the water at the base of low precipitous rocks. In addition to the reticulated masses of limestone covered with calcspar from the fallen cliffs, the search disclosed small tabular pieces of minutely crystallized quartz and angular masses of a kind of striped hornstone, gray and lead colored, which had been liberated from similar positions in the cliffs. On passing the west margin of the island, I observed a bed of a species of light-blue clay, which is stated to part with its coloring matter in baking it, becoming white. While the British possessed the island, they attempted to procure water by digging two wells at the site of Fort George (now Holmes), but were induced to relinquish the work without success, at the depth of about one hundred feet. Among the fragments of rock thrown out, are impressions of bivalve and univalve shells, with an impression resembling the head of a trilobite. These are generally in the condition of chalcedony, covered with very minute crystals of quartz. I also discovered a drift specimen of brown oxide of iron, on the north quarter. This sketch embraces all that is important in its mineralogical character.