Dana ' holds that Westchester County is comprised within t\u Green Mountain region, that it borders the southern side of the Putnam County Archa'an, as Dutchess County does the northern, and resembles in its ordei that part of the Green Mountain region which now makes Western Connecticut. The topographical features of the county owe much to the lime-stont bolts, which, by their easy erosion, have determined the courses of river valleys, and the lines of marshes along such valleys, as well as located many of tlu lakes. The beds of this soft rock stand nearly vertical, thus favoring the excavation of deep channels.

> PH|ier oil the Geolugical Relations of the Liue-atooe Belts of WestcUester County, N. Y., American Joitntalof Ixience.

The valleys are sometimes abrupt on both sides, but usually have one side high, precipitous and rocky, and the other gently sloping ; and this is largely due, in connection with the erosion, to the pitch or dip of the beds. But the i)itch of the beds may not have been the only cause of the form of the valleys. Prof. Dana holds that the throw of the waters against the right bank of a stream (the western if flowing south, or the northern if flowing west), in consequence of the earth's rotation, may have had its effects, and may possibly account for the cases in which the western side is the steep one, notwithstanding a vertical or even a high eastern pitch.

The lime-stone belts of the county are divided by Prof. Dana into: 1st, the Southern section of the county, from New York Island to White Plains ; 2d, The 3Iiddle section, from White Plains to Croton Lake ; and 3d, the Northern section, north of the Croton Lake.