270. THE FORMATION 
II. Anomalous successions: xenodochae (£éos, strange, unusual) 
327. Illustrations. The following series will illustrate the application of 
this system: of nomenclature to particular successions, and their stages, or 
formations. 
Thlaspi-Picea-sphyrium: pennycress-spruce talus succession 
Thlaspi-Eriogonum-chalicium: pennycress-eriogonum gravel slide forma- 
tion. 
Elymus-Gilia-chalicium: wildrye-gilia half gravel slide formation 
Quercus-Holodiscus-driodium : oak-fringewood dry thicket formation 
Pinus-xerohylium: pine dry forest formation 
Picea-Pseudotsuga-hvlium: spruce-balsam forest formation 
Bryum-Picea-pyrium: moss-spruce burn succession 
Bryum-telmatium: moss meadow formation 
Aster-Chamaenerium-poium : aster-fireweed meadow formation 
Deschampsia-Carex-poium: hairgrass-sedge meadow formation 
Salix-Betula-helodrium: willow-birch meadow thicket formation 
Populus-hylium: aspen forest formation 
Picea-hylium: spruce forest formation 
Lecanora-Carex-hedium : lichen-carex residuary succession 
Lecanora-Gyrophora-petrium : crustose lichen rock formation 
Parmelia-Cetraria-chalicium: foliose lichen gravel slide formation 
Paronychia-Silene-chalicium: nailwort-campion gravel slide formation 
Carex-Campanula-coryphium: sedge-bluebell alpine meadow formation 
Eragrostis-Helianthus-xerasium: eragrostis-sunflower drainage succession 
Eragrostis-Polygonum-telmatium : eragrostis-heartsease wet meadow for- 
mation 
Helianthus-Ambrosia-chledium : stinflower-ragweed waste formation 
INVESTIGATION OF SUCCESSION 
328. General rules. The. study of succession must proceed along two 
fundamental lines of inquiry: it is necessary to investigate quantitatively 
the physical factors of the initial stages and the reactions produced by the 
subsequent stages. This should be done by automatic instruments for 
humidity, light, temperature, and wind, in order that a continuous record 
may be obtained. Water-content is taken daily or even less frequently, while 
soil properties, and physiographic factors, altitude, slope, surface, and ex- 
posure are determined once for all. It is equally needful to determine the 
development and structure of each stage with particular reference to the 
adjacent formations, to the stage that has just preceded, and the one that is 
