THE GEOLOGY OP GIBEALTAE. 



535 



Mean. 



Minimum during six years, 

 1853-59. 



January . . 54*7 



37-9 



February 54*2 



35-3 



March . . 56-8 



41-2 



April .... 61-5 



47-9 



May .... 65-7 



49-3 



June 71-0 



48-6 



July .... 74-9 



60-8 



August . . 75-5 



61-5 



September 72-4 



55-9 



October . . 65-6 



50-9 



November 60*7 



44-6 



December 56*2 



32*7, lowest minimum in six years, 



Year 64-1 





It will be observed that the coldest month of the year is February, 

 which has a mean temperature of 54°-2. Mr. Buchan remarks that 

 an increase of 1000 feet to the height would lower the monthly 

 means about 3°, so that we may take the mean temperature of 

 February at the top of the Rock (1396 feet) to be in round numbers 

 50°. Let us now add another 500 feet to the height, so as to give an 

 altitude of, say, 1900 feet. An elevation to this extent would 

 restore the land-passage by which the southern Mammalia may have 

 entered Spain, but it would be quite inadequate to bring about cli- 

 matic conditions sufficient to account for the formation of the great 

 limestone-agglomerate. In point of fact it would reduce the mean 

 temperature of the coldest month by only 4° or 5°, thus giving a winter 

 mean of, say, 45° at the top of the Rock. Under such conditions, 

 the minimum temperature of 32° would no doubt be reached more 

 frequently than it is at present. But it is hardly necessary to say 

 that this would be altogether insufficient to induce the formation of 

 agglomerates like that of Buena Vista. To wedge-out blocks measuring 

 12 feet and more across, and to form thick masses of angular debris, we 

 need a frost capable of penetrating to some considerable depth ; and, 

 more than this, we require the agency of heavy snows, the melting of 

 which shall saturate the debris-heaps so as to cause them to flow en 

 masse for more than 500 yards over a gently inclined surface ; and 

 since much of the agglomerate has been formed by the splitting-up 

 of the limestone at levels which are now raised only some 300 feet 

 or so above the sea, it follows that a mean winter temperature not 

 above the freezing-point must have obtained even at these com- 

 paratively low levels. But before such a temperature could be 

 experienced now at Windmill Hill and Europa, we should require 

 the whole Rock to be upheaved for 6000 or 7000 feet. Surely we 

 can hardly suppose it possible that the Rock stood some 7000 or 8000 

 feet above the sea-level of Pleistocene times ! And if such a degree 

 of elevation be necessary to account for the phenomena of the mas- 

 sive limestone-agglomerates, then it is extremely improbable that 



