io66 PRINCIPLES OF STRATIGRAPHY 



Terrestrial Relicts. These are faunas and floras which have 

 migrated into certain regions during a period of dilTerent chmatic 

 conditions, and then became stranded in certain spots where tlie 

 environment remained uniform, while the surrounding area 

 changed. The Arctic plants and butterflies of Mount Washington 

 in New Hampshire are examples. These occupied the region dur- 

 ing the general refrigeration of the climate in the last glacial pe- 

 riod, and on the retreat of the ice were left stranded on the higher 

 points where these conditions were more nearly normal for them. 

 According to Professor Asa Gray, T^y northern species of plants 

 still remain and thrive on the summits of the White Mountains of 

 New Hampshire, and part of them also on the Adirondack and 

 Green Mountains. A number of these species were found on 

 a Greenland nunatak by the Jensen expedition of 1878. 



Dwarf Faunas and Micro-faunas. 



These are faunas in which all the individuals are of much 

 smaller size than their norm, and which therefore clearly lived 

 under conditions preventing the organisms from reaching their 

 full size. That such conditions are primarily environmental is 

 indicated by the fact that all the organisms are affected, whereas, 

 if individuals only were dwarfed, or only members of one species, 

 this might be considered as more likely an individual response. 

 Micro- faunas are associations of small species determined by the 

 peculiarity of physical conditions and not due to dwarfing of 

 normally large species. While dwarfed forms of known larger spe- 

 cies are readily enough recognized, it is not always easy to say 

 whether a given fauna — not known elsewhere — is dwarfed or is a 

 micro-fauna due to selection. 



Shimer (45) has listed the cliief agencies of dwarfing noted 

 in recent and fossil marine invertebrate faunas, as follows : 



1. A change in the normal chemical content of the sea water. 



(a) Due to freshening of the sea water. 



(b) Due to a concentration of the salt, iron, etc. 



(c) Due to an increase in H^,S and other gases. 



2. Presence of sand and other mechanical impurities in the 



water. 



3. A floating habitat. 



4. Variation in temperature. 



5. Extremes in depth of water and variation in amount of 



water per individual. 



