66 GEOLOGICAL MEMOIRS. 



arms and the powers which association gives him, he is unable to ex- 

 terminate their h\ang analogues. This only is certain, that the greater 

 number of these species were not driven to other climates, but were 

 destroyed by causes which it is difficult to appreciate. 



M. Pomel then gives geological details of the sequence of formations 

 as far as they can be traced. They admit of three divisions which 

 may be designated by the name of the prevailing rock, viz. trachyte, 

 basalt, and lava; the basalt having moreover triturated and worn down 

 the trachytes, so as tohave formed outof them pumiceous conglomerates. 

 Now as Anciat has been enumerated by M. Pomel among the localities 

 containing diluvial species in a pumiceous conglomerate, and as Pliocene 

 species occur in a similar deposit on Mont Perrier, doubts had arisen 

 as to the correctness of this identification, which M. Pomel proceeds 

 to remove. The oldest basalts of the Issoire are those of Pardines, 

 Broc, Perrier, &c., which repose on the lacustrine beds, or on alluvia 

 containing quartz pebbles lying on trachyte. Against these basalts 

 abut alluvia with volcanic debris, alternating with pumiceous tuffs 

 containing immense blocks of trachyte. The plateau of the Col de 

 Bonhomme is a detached part of the same conglomerate, as is proved 

 by the alternations being the same. They form almost the whole top 

 of the mountain, except on the north side, where a dyke occurs. The 

 slope of the mountain on the north side is uniform and pretty abrupt : 

 at its foot stands the small town of Champeix ; and about halfway 

 up are the remains of a bed of conglomerate at a much lower level 

 than the lowest beds of the upper conglomerate : on the east side this 

 bed is seen to abut against the slope of the soil which forms the base 

 of the upper. The relations therefore between these two beds of 

 conglomerate are the same as between the basalt and conglomerate 

 of Mont Perrier : the lower bed, abutting against a slope formed 

 subsequent to the deposition of the upper, must be the more recent. 

 It is impossible to explain away this result by supposing the upper con- 

 glomerate to have been separated from the lower by an elevation of the 

 soil ; since the granite which forms the basis of the country is of 

 nearly the same height on the two sides of the valley, and even slightly 

 lower on the side where it passes under the north of the Bonhomme. 

 Had the two deposits been basalts, they might by possibility be synchro- 

 nous ; the viscous nature and the cooling down of the border of the 

 sheet of basalt arresting it even upon the edge of a slope ; but it is a 

 dynamical absurdity to make such a supposition for alluvia formed by 

 running water at such different levels. These conglomerates therefore 

 on the slope of the hills above Champeix are newer than those of the 

 Croix de Bonhomme, which are identical with those of Pardines and 

 Perrier ; consequently newer than the basalts which these last contain ; 

 and a fortiori than the basalts which these last cover. M. Pomel 

 includes in the locality of Anciat all this newer tufaceous conglomerate 

 which extends fromMontaigu-le-Blanc to beyond Coudes-Montpeyroux 

 on the left bank of the Allier near its confluence with the valley of 

 Champeix or Neschers. On the east of Perrier is a deposit related 

 to that mountain as the Champeix conglomerates to those of the Bon- 

 homme : it comprises the localities of Malbatu, Peyrolles, and Tormeil. 



