206 Report of the State Geologist. 



But these Cryptogamia are not the only plants of the Carbo- 

 niferous epoch ; the Gymnospermia, already represented by the 

 Cycads and Cordaites, furnish us with more exact information 

 regarding climate.* It is known that the Dicotyledons and 

 Gymnosperms in the structure of their stems and roots present 

 traces, recorded continuously, of the influence of annual climatic 

 variations. 



Every yearly deposit of wood consists of an interior porous 

 layer, formed in the spring, and a more dense external one, pro- 

 duced in the autumn ; the thickness of the entire layer varies 

 moreover, according as the year was favorable or otherwise. 

 The differences of the annual layers are slight v^hen the climate is 

 uniform, and they indicate nothing more than periods of humidity 

 and dryness. 



But in examiniDg the trunks of the Conifers, at epochs more 

 and more remote, we find that the tissue becomes more and more 

 homogeneous, and at the coal epoch, the lines of demarcation are 

 scarcely indicated ; it is then especially at that epoch that the 

 climate must show the greatest uniformity. 



To sum up ; actual researches reveal a strongly marked tendency 

 to reduce the great differences which were thought to have 

 existed between the coal epoch and the present. S'evertheless, 

 it remains conceded that the climate must have been very 

 warm, as is shown by the coral reefs v/hich exist in the same 

 localities as during the Devonian, and which are also found as 

 far north as Nova Zembla and Spitzbergen. 



Climate of the Jurassic. — At the Permian and Triassic epochs, 

 the differences in the faunas are mostly in the pelagic and littoral 

 facies. Little is known concerning the climate of those periods. 

 It is during the Jurassic that we find, for the first time, certain 

 proofs of the existence of climatic zones. The marine fauna 

 is distributed in seas, the contour and facies of which are 

 relatively well known, so that we know how much to attri- 

 bute to the influence of temperature. An arctic zone is indi- 

 cated principally by the absence of Ammonites of the groups 

 of Lytoceras, Phylloceras and Simoceras. by the presence of 

 Acephala of the genus Aucella^ by the frequency of certain 

 Belemnites {B. excentricus) and the absence of Corals. This cold 

 sea sends arms toward the south, the most important of which 

 is the basin of Moscow, which communicates by straits with 

 a vast mediterranean sea, in which the terranes of Western 

 Europe appear as an archipelago. This interior sea is divided 

 into two parts by the limit of the climatic zones. The northern 

 portion forms the transition between the arctic zone and the 

 southern region. This latter presents the facies called Alpine, 

 extending through southern France, Spain, Italy, the Alps, the 

 Carpathians, and the Dobroudja; it has its southern limit in 



♦Renault, Cours de Botanique fossile. 



