Mr. R. Spruce on the Musei and Hepatice of the Pyrenees. 87 
ascending therm are usually small, and occupied either by lakes, 
or by alluvium deposited by the descending streams. In only 
two cases have I seen hollows filled with peat, one on Mont 
Goursi in the Basses Pyrénées, and the other at the head of a 
small yalley, lateral to the Vallée de Lesponne in the Hautes 
Pyrénées. 
The line of perpetual congelation in the Pyrenees, I assume 
from my own observations to be at an average height of nearly 
9000 feet, or more than 1000 feet higher than in the Alps. One 
authority, now before me, fixes it at 8718 feet, and Ramond 
estimated it at from 8100 to 8400 feet, which I do not hesitate 
to say is much too low. It varies however considerably with the 
degree of exposure and even with the form of a mountain, and 
the snow is uniformly found to melt less, and consequently to 
- descend lower in an eastern exposure than elsewhere. Hence, 
even on the highest mountains, the band of perpetual snow is 
not more than from one to two thousand feet broad. 
The streams which take their rise on the southern slopes of 
the Pyrenees flow nearly all into the Ebro. On the northern 
slopes, the space lying opposite the western half of this drainage 
of the Ebro is occupied by the Adour and its tributaries, while 
the space corresponding to the eastern half, extending from the 
source of the Adour to that of the Arriége, is occupied by the 
upper part of the basin of the Garonne. In the extreme eastern 
angle, on both the northern and southern side, are various small 
streams which run directly into the Mediterranean. This drain- 
age of the rivers would seem to afford us the basis of a division 
of the Pyrenees, for the purpose of estimating the distribution of 
plants on their surface ; but on trial such a division will be found 
imtractable, and I prefer another which separates the plants into 
more distinct groups, and corresponds very nearly with that 
adopted by the botanistes sédentaires of the Pyrenees. I divide 
the Pyrenees into three districts, the Western, the Central, and 
the Hastern, the limits of which I proceed to define. 
The Central Pyrenees are comprised between the upper part 
of the Gave de Pau, from its source at the Cirque de Gavarnie 
as far as to the bridge of Lourdes, on the west; and Mont Mala- 
detta and the Vallée d’Aran, watered by the infant Garonne, on 
the east; or from the meridian of Greenwich* to about 50 minutes 
of east longitude. This district includes, in France, the upper 
part of the Dept. of the Haute Garonne and most of the upper 
part of the Hautes Pyrénées ; in Spain, part of Aragon and a 
very small angle of Catalonia. It is watered by the upper 
* The village of Luz, in the valley of Baréges, is exactly in the longitude 
of Greenwich. 
