4. _Agate._--Is found with the preceding. It is more frequently found in larger masses, in the rock, which are sometimes spheroidal, reniform, or globular. These agates are chiefly arranged in concentric layers, which are white, red, yellow, &c., according to the colors of the different varieties of chalcedonies, carnelians, &c., of which they are composed. A close inspection would also separate them into several varieties--as onyx, agate, dotted agate, &c. Subs. 6.--_Hornstone._ In nodular or angular masses, imbedded in the secondary limestone of the west shores of Green Bay; and in the beds of argillaceous white clay strata of Cape Girardeau, of Missouri. Also, on the hills of White River, Arkansas. Subs. 7.--_Jasper._ 1. _Common Jasper._--In detached fragments, yellow, in the drift of Lake Superior. 2. _Striped Jasper._--With the preceding. Most commonly, these specimens consist of alternate bands of red and black, or brown. 3. _Red Jasper._--In quartz rock, Sugar Island, River St. Mary's, Michigan. Masses of this mineral have been met in situ. Subs. 8.--_Heliotrope._ A fine specimen of this mineral, now before me, was procured at the mouth of the Columbia River, Oregon. It is in the form of an Indian dart. Its color is a deep uniform green, variegated with small spots of red; those parts which are green being fully translucent, the others less so, or nearly opaque. This beautiful mineral is represented to have been in common use by the Indian tribes of the Northwest Coast, for pointing their arrows, previous to the introduction of iron among them. It differs chiefly from the dotted jaspers of Lake Michigan, in its translucence and green color. Subs. 9.--_Opal._ Common opal occurs as a constituent of agate, along with chalcedony rarely, in the drift on the south shore of Lake Superior. 2. SILICIOUS SLATE. 1. _Common._--In subordinate beds, in the argillite of the River St. Louis, northwest of Lake Superior. 2. _Basanite_ (_Touchstone_).--In detached fragments in the drift on Lake Superior, and along the banks of the Upper Mississippi generally. 3. PETROSILEX. In large isolated masses in the bed of the Illinois River, on the shallow rapids between the junction of the Fox and Vermilion Rivers. It is mostly arranged in stripes or circles of white, gray, yellow, &c., resembling certain jaspers, or approaching sometimes to hornstone. The bed of the Illinois River, at this place, is a species of gray sandstone. Also, in detached fragments, on the south shore of Lake Superior, intimately mixed with prehnite. In regard to the latter, Professor Dewey, of Williamstown College, writes me: "I have received from Dr. Torrey, a curious mixture of petrosilex and prehnite, in imperfect radiating crystals, which was sent him by you and collected at the West. He did not tell me the name, but examination showed what it was. The association is singularly curious." The locality of this mineral is Keweena Point, Lake Superior. 4. MICA.