Mr. R. Spruce on the Musci and Hepaticcp of the Pyrenees. 87 



ascending them 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 Pyrenees, and the other at the head of a 

 small valley, lateral to the Vallee de Lesponne in the H antes 

 Pyrenees. 



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 Arriege, 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 

 intractable, and I prefer another which separates the plants into 

 more distinct groups, and corresponds very nearly with that 

 adopted by the botanistes sedentaires of the Pyrenees. I divide 

 the Pyrenees into three districts^ the Western, the Central, and 

 the Eastern, 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 Vallee 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 Pyrenees ; in Spain, part of Aragon and a 

 very small angle of Catalonia. It is watered by the upper 



* The village of Liiz, in the valley of Bareges, is exactly in the longitude 

 of (irecnwich. 



