The Adirondacks are not part of the Appalachian Mountains, despite being just across Lake Champlain from the Green Mountains and just across the Mohawk River from the Catskills, both Appalachian ranges. Aside from the broad valleys that separate the Adirondacks from its neighbors, the geology of the range is very distinct--steep, rocky slabs of exposed bedrock and a more jumbled topography give the Adirondacks the same general wild, rugged character and feeling of the Canadian Shield territory to the north.
So, even though the U.S. Geological Survey's map of physiographic regions groups the Adirondacks in the "Appalachian Highlands Division", and the range lies well south of the rest of the Canadian Shield, classifying them as part of the Appalachian Mountains is just plain wrong. So this site places the triangle of land between the Saint Lawrence, the Mohawk, and Lake Champlain as a dangling outlier of the huge, geologically similar belt of the lowlands and hills stretching across Canada.
The Adirondacks, especially the High Peaks area, are perhaps the most consistently high, wild, and remote mountain area in the eastern United States. The White Mountains may be higher, some of Maine's peaks more remote, and both states's ranges more extensive, but only the Adirondacks combine a huge area of rugged mountians and remote wilderness. Trails here are generally steeper and more rugged than those in New England, and about half of the 50 highest peaks don't even have maintained trails. Some of the higher peaks are so far from roads that many hikers are not able to do them on dayhikes, a situation almost unheard of in the most Appalachian ranges.
Most of the Adirondacks remained an unexplored wilderness well into the mid-1800s, when huge parts of the Rockies had already been well-mapped.