Rock county lies west of Walworth, and north of the Illinois line, with Rock river running through it north and south, nearly in the center. Its population in 1846 was about 7,000; it is now above 20,000; dwellings, 3,631; farms, 1,975 ; manufactories, 126. In quality of soil and other agricultural facilities it is not surpassed by any county in the state; while its water-power is probably equal to any. It embraces some of the best prairies in the state, a large portion of which is prosperously cultivated. Deer lake, and the south end of Kashkanong [Koshkonong], are the chief lakes in this county, with several very small ones. Rock river and Turtle creek are the main streams in Rock county. JANESVILLE is the county-seat. It contains 3,500 inhabitants, and is rapidly growing. Rock river affords ample water-power, on which is already built several fine mills, and other machinery. The village occupies both sides of the river; and equals most towns in the state in activity of business. Mr. LAPHAM thus describes its admirable location in 1846:   "It is situated on a flat, or level, between the river and the foot of the bluffs, which are about one hundred feet high. The courthouse is erected on the bluff, giving it a very prominent appearance. Janesvffie is the point at which much of the trade between the eastern and western portions of the Territory crosses Rock river, and a bridge is now erected for its accommodation. The distance from Janesville to Milwaukee is sixty-five miles, and the same to Racine; giving the citizens a choice of two ports on Lake Michigan, reached in the same distance; it is 13 miles from Beloit, 41 from Madison, 31 from Monroe, and about 80 from Mineral Point."   A Railroad Company is chartered, the stock subscribed and the work commenced in good earnest, to run a line from Fon du lac to this place; thence, southeast through Walworth county into Illinois, and on to Chicago; which will be speedily constructed. Good building and limestone is quarried here in abundance. BELOIT is another very beautiful and flourishing village in this county, situated on both sides of Rock river, at the junction of Turtle creek; its population is about 3,000. It is a place of active and increasing business; and is noted for its elegant buildings, and fine wide streets. It has several excellent mills, machine shops, and a prosperous college. Here, I am told, was erected the first flouring mill in the State. Its water-power is valuable. The town is located on a level plain, but is fast extending on to the bluffs each side of the river. The college is erected on a high and airy bluff, commanding an extensive and varied prospect. In this vicinity may be seen many of those singular and mysterious mounds, which abound in the West. Here, as at most other towns on Rock river, abundance of good stone is quarried, valuable alike for lime and building purposes. This place is noted for its fine churches. The stock has been subscribed, and arrangements made for having a branch from the Galena and Chicago Railroad built to Beloit, which will be rapidly completed. There are several other small villages in this county, among which are Fulton, Clinton, Milton, Johnstown, Waterloo, etc.

Mr. Thompson's Letters. The following are extracts of letters, written in June and July last; in copying from these letters I have omitted what was not of general interest.   "The scenery of the Rock river corresponds more nearly with that of Massachusetts and Western New York than any other in the Western country. The long-extended bluffs, of various hight [height], resemble the hilly banks of a New England stream, the bottom of the river is pebbly and the water clear and bright, and the banks are well covered with groves. And then you have what New England nowhere affords, the prairie, the beautiful prairie, not so vast as to be overpoweringly dull and tame, but large enough to be novel and wonderful to eastern eyes. The difference between the make of Wisconsin and Illinois is given, in the fact that in Wisconsin the prairies are named, and in Illinois the groves. "In the southern border of Wisconsin, just across the line of Illinois, on the bank of the Rock river, stands BELOIT, a town of some fifteen hundred inhabitants (nearer three thousand), and the seat of a College which is sustained in part by the College Society. I hardly knew the town when I alighted from the stage, so greatly has it changed in six years. It spreads along both sides of the river, and is laid out with much taste. When I was here in 1845, I went up on the highest bluff upon the eastern bank to examine some Indian mounds and to enjoy the view of the rolling prairie stretching southward into Illinois. This bluff was then talked of as a site for a college, and Rev. Mr. SQUIER, of Geneva, had just been on the ground and had made liberal proposals for the endowment of such an institution. (Thanks to the kind nursing of that gentleman during that trip, I am now alive to write this letter, having greeted him as a professor in his adopted College.) Now upon that same bluff, sheltered by its lofty grove, and beside the undisturbed mounds of other days, stands a College edifice of more imposing architecture and of better adaptation to the wants of such an institution than any college building I have seen in the West. This edifice, substantially built of brick, is about a hundred feet long by forty in depth, four stories high, with lofty ceilings, spacious and well ventilated rooms for recitations and lectures, and several good dormitories in the fourth story. This is intended for the main college building, to be hereafter flanked with corresponding wings. It was erected by the citizens of Beloit at a cost of about $12,000. "Beloit College is already in vigorous operation. The decorum of the students, and the general order of the institution are worthy of all praise. "The ride from Beloit to Janesville in a buggy by night, along the lately flooded bank of the river, through deep sloughs and over bridgeless creeks, was somewhat perilous, and on reaching Janesville, a stage-driver congratulated us that we had got safely through a ditch that he avoided by day by a circuit of three miles! One collocation of incidents interested me greatly. Soon after leaving Beloit at sunset, we came upon an encampment of emigrant wagons near some Indian mounds; there were the tombs of the old savage occupants of this rich soil, there were the eager travelers from the Old World coming to find a home in the New, there stretched the telegraph wire, the symbol of a far-reaching civilization, and yonder loomed the college which should mould these raw materials and shape them into a cultivated religious society. "JANESVILLE is already a town of considerable trade, one of those inland river towns that every State requires as a central depot. It has now one of the best hotels in the State and many fine stores. The ride from Janesville through Rock, Walworth, and Milwaukee counties, though it exhibited the riches of the country—'as good land as ever lay out doors'—was any thing but comfortable to a spare body already jolted by two days' staging. That Troy marsh and the corduroy make one ache for the completion of plankroads."