ifo. 1.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 65 



The narrow district occuined by the Pacific forest has a much more uncriual rainfall, 

 more unequal in its different parts, most unequal in the different seasons of the year, 

 very different in the same place in ditfei-ent years. 



From the Gulf of Mexico to the Gulf of 8t. Lawrence the amount of rain decreases 

 moderately and rather regularly from south to north ; Itut, as less is needed in a cold 

 climate, there is enough to nourish forest throughout. On the Pacific coast, from the 

 Gulf of California to Puget Souud, the southerly third has almost no rain at all ; the 

 middle portion less than our Atlantic least ; the northern third has ahoiit our Atlantic 

 average. 



Then, New England has about the same amount of rainfall in winter and in summer ; 

 Florida and Alabama about one-half more in the three summer than in the three winter 

 months — a fairly equable distribution. But on the Pacific coast there is no summer 

 rain at all, except in the northern portion, and there little. And the winter rain, of 

 forty-four inches on the northern border, diminishes to less than one-half before reach- 

 ing the Bay of San Francisco ; dwindles to twelve, ten, and eight inches on the 

 southern coast, and to four inches before we reach the United States boundary lielow 

 San Diego. 



Taking the whole year together, and confining ourselves to the coast, the aveiaige 

 rainfall for the year, from Puget Sound to the border of California, is from eighty 

 inches at the north to seventy at the south, i. e., seventy on the northern edge of 

 California ; thence it diminishes rapidly to thirty-six, twenty (about San Francisco), 

 twelve, and at San Diego to eight inches. 



The two rainiest regions of the United States are the Pacific coast north of latitude 

 forty-five, and the northeastern coast and borders of the Gulf of Mexico. But when 

 one is rainy the other is comparatively rainless. For while this Pacific rainy region 

 has only from twelve to two inches of its rain in the summer months, Florida, out of 

 its forty to sixty, has twenty to twenty-six in summer, and only six to ten of it iu the 

 winter months. 



Again, the diminution of rainfall as we proceed inland from the Atlantic and Gulf 

 shores, is gradual ; the exj)anse that is or was forest-clad is very broad, and we wonder 

 only that it did not extend farther west than it does. 



On the other side of the continent, at the north, the district so favored with winter 

 rain is but a narrow strip, between the ocean and the Cascade mountains. East of the 

 latter the amount abruptly declines — for the year from eighty inches to sixteen ; for the 

 winter mouths, from forty-four and forty to eight and four inches ; for the summer 

 months, from twelve and four to two and one. 



So we can understand why the Cascade Mountains abruptly separate dense and 

 tall forest on the west from treelessness on the east. We may conjecture, also, why 

 this North-Pacific forest is so magnificent in its development. 



Equally, in the rapid decrease of rainfall southward, in its corresponding restric- 

 tion to one season, in the continuation of the Cascade mountains as the Sierra Nevada, 

 cutting off access of rain to the interior, in the unbroken stretch of coast ranges near 

 the sea, and the consequent small and precarious rainfall in the great interior valley 

 of California, we see reasons why the Califomiau forest is mainly attenuated south- 

 ward into two lines — into two files of a narrow but lordly procession, advancing 

 southward along the coast ranges, and along the western flank of the Sierra Nevada, 

 leaving the long valley between comparatively bare of trees. 



By the limited and precarious rainfall of California, we may account for the limi- 

 tations of its forest. But how shall we account for the fact that this district of 

 comparatively little rain produces the largest trees in the world f Not only produces, 

 alone of all the world, those two peculiar Big-trees which excite our special wouder — 

 their extraordinary growth might be some idiosyncracy of a race — but also produces 

 Pines and Fir-trees, whose brethren wo know, and whose capabilities we can estimate 

 upon a scale only less gigantic. Evidently there is something here wonderfully favor- 

 5 GB 



