OCT., 1899.] ST.OPE EXPOSUHE. 47 



the sun, which became (hu'i) red like the full inoou and then disappeared. 

 At 5 o'clock the smoke began to settle back, as it always did when the 

 chill of the evening- came on, and the sun reappeared, to set as usual 

 behind the dark ontlines of the distant mountains. 



Fires on the south, in the valley of the McCloud, cut oft' the view iu 

 that direction, and it was only at rare intervals, and nsually at sun- 

 set, that we were able to see the snowy crown of Ijassen Butte <)0 

 miles away. Even Castle Crags, almost at our feet, weie rarely visible. 

 This experience is frecinent in the west; and of the hundreds of persons 

 who visit the Pacific slope every summer to see the mountains, few see 

 more than the immediate foreground and a haze of smoke which even 

 the strongest glass is unable to penetrate. 



Along the railroad between the head of Sacramento Canyon and 

 Shasta Yalley one traverses desolate tracts which a few years ago were 

 covered by a noble forest of ponderosa and sugar pines. 



EFFECTS or BITRNS IN CHAN(rING ZONE POSITIONS. 



A burn in the lower part of the Shasta fir forest a little above 

 Wagon Camp affords an excellent illustration of the way fires some- 

 times change the zone relations of particular areas. The area iu ques- 

 tion was well within the Canadian zone before the fire, which occurred 

 only a few years ago. Since the fire. Transition zone species have crept 

 up the ridge, and now Geanofhns reJufinus, Arciosiapliylos paUda^ Ror- 

 Ivelia pseiulocapitata, Aiwciinnm piim il urn , (r((yophi/tHmr<(mofii.sf>im am, aud 

 others are common. The manzanita and buck-brush are young and 

 small but are spreading, so that in a few years the ridge, whi<,'li has a 

 warm southerly slope, will be mainly Transition. But in the mean- 

 time a new growth of Shasta firs has started, and in ten or twenty 

 years is likely to overtop and drown out the Transition zone species, 

 enabling the Canadian zone to reclaim the burn. 



Such cases of alternation of zones resulting fioni the clearing of 

 forest land are not uncommon, i)articularly when deforestation is 

 caused or accompanied by fire. But on steeper sloi)es, especially rock 

 slopes, if the vegetable layer is burned off, the (lower) zone wliich 

 creeps up to replace the (higher) one destroyed becomes permanent or 

 nearly so. 



It may be laid down as a general rule that the destruction of forests, 

 by admitting the sun and wind, lessens the moisture in the soil and 

 increases the temperature, thus inviting animals and plants to come in 

 from adjacent warmer areas. Deforestation of an area therefore tends 

 to lower its zone position. 



SLOPE EXPOSURE. 



By slope exposure is meant the inclination of the earth's surface at 

 a particular point with reference to the angle at which it receives the 

 sun's rays. The amount of heat, were it not for the retarding effect of 



