THE GRAND EXCURSION of 1854 brought 1200 people to the edge of the world. Of course, they knew the actual world went far beyond the  Mississippi River. Retracing the route of the Grand Excursion is an exhilarating experience. The grand vistas, picturesque islands, and awe- inspiring bluffs are still there. Eagles still soar overhead, and waterfowl continue to nest in the sloughs and backwaters. But today's skyline along the shore is as much shaped by steeples and smokestacks as by willow and cottonwood trees. In the space of 150 years, the river has spawned the flourishing cities that today line its banks and pay tribute to its nurturing presence.