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An unexpected concentration of calcicole mosses and lichens has been discovered at and near a small spring seep on the granite headwall of the North Basin of Mt. Katahdin, Piscataquis County, Maine. Water samples from the spring were circumneutral and high in calcium ion content, whereas other springs, ponds, and streams on the mountain tested acidic and low in calcium. Because the Katahdin granite contains no primary carbonate minerals, it is suggested that the source of calcium enrichment in the North Basin spring water is secondary calcium carbonate (calcite) precipitated in cavities and fractures near the base of the upper Katahdin granite and its subsequent dissolution and mobilization by percolating groundwater. Plagioclase, epidote, and other calcic minerals in the granite supply the primary calcium ions. The moss calcicoles grew in small discontinuous patches on wet humified peaty soil over alpine bedrock ledges in communities having floristics and water chemistry similar to lowland rich fens. Present in or near the seep were mosses previously unrecorded for New England or Maine [Loeskypnum wickesiae (Grout) Tuom., Neckera oligocarpa Bruch in Hartm., Pseudoleskea radicosa (Mitt.) Mac. & Kindb., Tortella tortuosa var. fragilifolia (Jur.) Limpr., Warnstorfia sarmentosa (Wahlenb.) Hedenäs], one lichen [Hymenelia cyanocarpa (Anzi) Lutzoni], reported here for the first time from North America, and another lichen (Thelidium minutulum Körb.) for the first time from the contiguous United States. An area adjacent to the seep, but unaffected by calcareous seep-water, was remarkable for a concentration of rare lichen species of acidic rocks, including Catillaria contristans (Nyl.) Zahlbr. and Lecanora caesiosora Poelt, that have not previously been reported from North America. The bryophyte and lichen flora of the seep area is an example of a Holocene alpine microrefugium. Refugia such as these can serve as a source of propagules that allow new populations to establish elsewhere in mountainous terrain, or beyond, during climatic and other environmental change. Therefore, extant, cryptic microrefugia are important biogeographically and especially worthy of preservation efforts.