Geologka I Society. 39 



From an examination of numerous escarpments, the details of which 

 are given in separate sections, the authors establish the following de- 

 scending order. . 



1 . Strong beds of white limestone, alternating with marls, and 

 containing the following fossils :—Limneus longiscatus, and others ; 

 Planorbis rotundatus, and cornu; Ancylus elegans, &c. 



2. White thinly foliated marls and marlstones, with a vast pro- 

 portion of flinty and resinous silex, both in layers and in nodules, the 

 latter freciuently having the characters of the menilite of the Pans 

 basin, containing innumerable Bulini, chiefly Bulini conicus and 

 pygmaeus, with Fotamides Lamarckii, and a great quantity of stems 

 of vegetables with gyrogonites. This middle system is distinguished 

 bv the pauer-like lamination of its beds 5 and from the succession of 

 matted vegetables and minute organic remains, it offers throughout 

 many striking analogies to deposits in recent lakes. (Some of the 

 thicker calcareo-siliceous beds are extensively worked for millstones.) 



•6. The base of these deposits is a brownish red plastic clay, charged 

 with white quartz pebbles, ^-c, the detritus being apparently derived 

 from the gneiss and mica schist, on which it rests. 



The united thickness of the lacustrine formations of the Cantal is 

 estimated at from 400 to 500 feet. 



Several detached remnants of water deposits are mentioned as oc- 

 curring between Aurillac and Mauriac ; and although the authors 

 conceive these may possibly have been formed in tarns (or small 

 lakes), yet from the prodigious convulsions which the whole country 

 has undergone posterior to the lacustrine deposits, it cannot be de- 

 termined whether these might not have been bays of the great lake 



of the Cantal. . , r , • 1 f*i- 



That a vast change in the relative levels of the various rocks of this 

 region has taken place, is proved by many of the escarpments of the 

 fresh-water marls being now at much greater heights than the border 

 primary rocks on which they rest. The mineralogical appearances of 

 the white limestone and marl are compared with the chalk of England, 

 like which their surface is occasionally hollowed out into root-shaped 

 cavities filled with alluvium ; while some of these fresh-water flints are 

 found strewed over the adjacent primary rocks, just as chalk flints are 

 spread over the granite of Peterhead, Banfl'shire. 



The valley of the Cer is then described. In ascendmg the deep 

 gorges of this valley to the Plomb du Cantal, or centre of igneous crup- 

 fion, the lacustrine strata gradually losing the honzontality vyhich 

 thev exhibit at Aurillac, arc found first much disturbed, then dislo- 

 cated, isolated and altered, amidst trachytic breccia and basalt; and 

 finally above Thiesac are entirely lost under the increasing moun- 

 tainous accumulations of volcanic matter. Siliceous Iragiucnts in- 

 closing fresh-water shells are found at such very high levels 111 some 

 of these ancient trachytic currents, and so much above any remnant 

 of the fresh-water strata m situ, that the autliors conceive tiicy must 

 have been ejected from below, and borne down Ircnn the central 

 heights of the volcano, mingled willi the detritus of volcanic rocks. In 

 confirmation of what has been previously stated, that the great vol- 



l 2 came 



