A similar alteration in the order of succession of forest growths was reported by the geologists employed by the Government in conducting the early surveys of Michigan. They stated that large tracts of land, originally covered with pines, had been succeeded by a second growth consisting of white birch, aspen, pine and hazel. In this case the primeval forest had been destroyed by high winds and afterwards burnt. It was supposed that where clearings had been made by fire, changes of the above nature occurred, but no suggestions were given as to the manner in which these new and strange growths established themselves. Near Ishpeming, I saw that in all cases where forest openings had been made by the action of fire, luxuriant masses of dense raspberry bushes occupied the land; and it was said that where the woods were cut down by the axe bushes of another class, bearing different berries, sprang up and flourished. In other parts of the North-west, variations in the succession of forest trees have been seen to occur; and usually there seems to be some connection between the type of the new plants and the methods by which the clearings have been made. As a general rule there are very few birds to carry seeds or pips from distant regions and therefore it is possible that in places where the ground has never for many centuries been previously disturbed and where the thick forest has been for the first time removed, and sunlight and fresh atmospheric conditions are admitted upon the land, new circumstances arise which are favourable to the development of dormant life.[41] Before leaving the valley of the Platte I made several excursions on the plains for the purpose of examining two of the most extensive of those singular groups of mounds which have been called prairie-dog villages. The largest of them was situated about two hundred miles west of the Missouri and three miles north of the Platte. It occupied a space exceeding fifty acres, which was covered by the rounded heaps of earth and pebbles thrown up by the little marmots when excavating their burrows. These creatures, when they are sitting on the tops of their mounds in a watchful attitude keeping a sharp look out around them and holding their paws before them, resemble a colony of ground squirrels. When I was at a certain distance from them they maintained continuous and defiant sounds like the shrill yelps of puppies, but upon a nearer approach they suddenly disappeared down their holes.