‚394 Part IV. Chapter 2. 
Xanthoxylum americanum, Celastrus scandens, Vitis cordifolia, Ampelopsis (Parthenoeissus) quinque- 
folia, Cornus circinata, C. sericea, C. candidissima, C, alternifolia?). 
2. Rock- and Talus Formations. 
Kock Gorge Formation. This formation is usually found on precipitous 
cliffs. The Dalles of the St. Croix and Wisconsin rivers may be taken as 
typic where rivers have worn deep gorges through the rocky strata. Ferns 
may be called the most characteristic feature in the St. Croix gorge. Growing 
in the clefts of the rocks is Dryopteris (Aspidium) fragrans, Woodsia obtusa, 
while on exposed faces occurs Woodsia ilvensis and on shaded cliffs Cystopteris 
Fragilis, Camptosorus rhizophyllus; Talinum teretifolium occurs on the diabasic 
rocks with Houstonia purpurea and Oxybaphus nyclagineus?). 
The cliff faces and damp, dark crannies of the Dalles of the Wisconsin show 
tufts of Gymnostomum, Pellaca atropurpurea, Dryopteris fragrans and on sur- 
faces protected by overhanging shelves, sheets of the rare sword moss, Bryo- 
ziphium norvegicum (= Eustichia norvegica). The sandy summits of the cliffs 
show patches of Mosses (Polytrichum, Dieranum, Leucobryum and Thelia 
Lescurü). The walls of certain of the canyons are occupied by Primula fari- 
nosa, Viola Selkirkii, while rooted in the crevices of more exposed sandstone 
places grows Potentilla Fruticosa, Rhododendron lapponicum and in similar more 
shaded situations near the water line Sullivantia ”; 
The dominant crevice herbs in general are: Campanula rotundifolia, Houstonia purpurea, 
Heuchera americana, Agrostis hyemalis, Arenaria strieta} the dominant crevice shrubs: Juniperus 
sabina, J. communis, Spiraea salieifolia, Rosa Woodsii, Diervilla triida; the crevice trees are: 
Pinus strobus, P. Banksiana, Q. macrocarpa, Fraxinus americana, Populus tremuloides. 
There are rock hills not connected with the present erosion of rivers. 
These hills have their peculiar vegetation. Stony Island is an outcrop of dolo- 
mitic Niagara (Silurian) limestone. The limestone is jointed and fractured and 
in the crevices occur mosses (Ceratodon and Bryum), while in the shallow soil 
on the rock face are Potentilla arguta, Verbascum thapsus, Heuchera hispida, 
Poa compressa. The opportunity for growth of shrubs eventually arrives and 
there appear Prunus virginiana, Physocarpus opulifolius, Rhus radıcans a0 
Iyphina, Rosa humilis, Ptelea trifoliata, Pirus coronaria. Later tree vegetation 
appears, . 
Talus Formation. Naturally the character of the talus depends upon the 
ledge from which it fell. Various types of vegetation occur on talus and form 
a°soclations of herbaceous, shrubby or arboreal constituents. Space will permit 
the description of only one or two characteristic associations®). 
1) UpHam, WARREN: Geographic Limits of Species of Plants in the Basin of the Red River 
of the North. Proceedings Boston Society of Natural History XXV: 140. 
2) Hırı, E. J.: Notes on the Flora of the St. Croix Region. Botanical Gazette XVI: 108, er 
3) True, Ropxey H.: Botanizing in the Dalles of the Wisconsin River. Plant World I: 81. 
March 1898. 
4) See in this connection MACMILLAN, C.: Observations on the Distribution .of Plants along 
