272 FORESTS AND TEEES OF NORTH AMERICA. 



geological boundary, between the metamorphic roclss of the hills and 

 those of later date, corresponding to the change in soil characterizing 

 the sandy flat low country. 



The line 7 — 8 is the northwestern boundary of G, and a very natu- 

 ral one both geologically and botanically. The range of animals also 

 corresponds to it in some degree. The boundary indicated by 9 — 10 

 is not a fixed limit, since the natural history of Florida is still but 

 little known. There is, however, a geological line near this point, 

 which, with the rapid increase of temperature southward, will proba- 

 bly be found to limit the greater number of species characteristic of 

 Florida. The line connecting this with 8 is also only arbitrary or 

 geographical, as it separates the waters of the Atlantic and of the 

 Gulf. There seem to be several trees_, however, limited in range near 

 it, and probably none can be made more natural. 8 — 12 is a continu- 

 ation of the line separating the low, sandy country from the granitic 

 and limestone formations of the hilly country. From 12 it extends 

 southwest, bordering the hilly table-land west of the Mississippi 

 valley. 



The hrohen line 6 — 12 is one of the most unsettled of all. Though 

 a number of trees are limited in this vicinity, there is no geological or 

 geographical line by which they may be bounded, and an attempt is 

 therefore made to make an average limit, depending on climate and 

 elevation as well as on the actual facts of distribution. 



An eminent geologist has suggested to me that perhaps the southern 

 limit of the drift may be found the true boundary here and on the 

 lineof 15— 16. 



The lines G — 6 — 15 — 16 may represent the summer isothermal of 

 about 75°, while 7 — 6 — 15 — 16 are near that of 35° for winter and 

 55° for the year. 



The numbers 11 — 15 — 13 indicate the eastern border of prairies, 

 and the limit of the great number of plants and animals which belong 

 exclusively to them. It is one of the best marked of all the lines, 

 though_, since the settlement of the country, it has become obscured 

 both by the extension of forests westward and their destruction east- 

 ward of it. The line represents nearly its natural position. The 

 figures 13 — 23 show the northeastern border of the prairies, which is 

 considered pretty well established, as far as authorities examined give 

 any data for it. Richardson mentions their northern extremity near 

 Great Slave lake. 



The line 13 — 14 — 22 is a good geographical boundary, and also 

 nearly coincides with the isothermal of summer 70°, though winter 

 seems to differ much at its two extremities. 14 — ^^18 is the line of 

 about 2,000 feet general elevation, and west of which scarcely any of 

 the eastern trees extend; in all not over six or eig;ht species, while, as 

 already stated, most of M and N is entirely woodless. 



The numbers 17 — 18 show approximately theboundary of the trop- 

 ical group of animals and plants which are found on the lower Rio 

 Grande. On a larger map the direction of this line would be more 

 irregular in detail. 



The numbers 18—19—16—21—22—23 mark the line limiting the 

 forests and other products of the western mountains. It will be found 



