426 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 22, 



substances, which altered the sediments then forming in the sea. 

 The cause must have been one which acted in a horizontal direction, 

 and not in a vertical one, as is usually the case in true plutonic action. 

 I conceive that cracks or vents may have opened at points distant 

 from each other, up which have escaped great volumes of gas accom- 

 panied by great heat, which was communicated to the triassic sea, 

 enabling it to alter chemically the deposits then forming, and favour 

 their crystallization even at points very distant from the orifices. 

 The occurrence of compact limestones alternating mth the crystalline 

 appears to be opposed to this hypothesis ; but it is easy to suppose 

 that the gaseous emanations took place repeatedly, after intervals of 

 repose, during which the sea tended to recover its temperature, ana- 

 logous to the periods of repose of modern igneous and mud volcanos. 

 This theory appears to me to account for the want of metamorphism 

 in the inferior beds, the alternations of crystalline and compact lime- 

 stones, and the passage from the compact to the oolitic structure 

 which is seen gradually to develope itself in the Jurassic rocks. The 

 oolitic structure, so common in volcanic regions and in the neighbour- 

 hood of thermal springs, according to my views is also due to thermal 

 causes, and has a direct relation to their intensity. The alterna- 

 tions of oolitic with compact beds indicate the periods of activity or 

 repose, and the gradual change observable in the higher beds points 

 to a gradual cooling-down of the temperature of our seas to the pre- 

 sent time. The crystalline limestones which appear to form the 

 lower limit of the Jurassic rocks are covered by others of oolitic 

 structure, which alternate with a grey compact limestone and with 

 conglomerates of limestone fragments. Fossils are rare in the cry- 

 stalline as well as in the oolitic limestone ; they only begin to be com- 

 mon in a grey, yellowish, or reddish limestone, intercalated among 

 the oolites, which contains casts of Trochi and Melanice, closely re- 

 sembling species which in England, France, and Germany are charac- 

 teristic of the lower oolite. The same limestone contains a new spe- 

 cies of Perna, resembling the Gervillia Benauxiana, D'Orb. Above 

 these are found grey shelly beds, together with one containing the 

 well-known Phytolites of Rotzo in the Sette Comuni. No one, as 

 far as I am aware, has studied these vegetable impressions, which I 

 propose to describe and figure in my work on the Alps : in the mean- 

 time I may state my opinion that this bed also is lower oolite. Marl, 

 alternating with ash-coloured beds, containing Terebratula ornitlio- 

 cephala, Sow., T. hullata, Sow., an Astarte, a Nerincea, and fossils in 

 bad preservation, announce the commencement of the middle oolite. 

 Here begins a white, yellow, and red Lumachello marble, in which I 

 have found no fossils except traces of an Astarte : it resembles the 

 marble of Arzo in Lombardy, where it contains Terebratula ornitho- 

 cephala. Above this Lumachello, which is very persistent through 

 our mountains, occurs the red, white, or grey limestone, which goes 

 by the name of Ammonite limestone (calcaire ammonitifere), from 

 the enormous number of Ammonites which swarm in its beds. In 

 the basin of Trent, in the mountains forming the valley of the Adige, 

 in the basin of Roveredo, in the high mountains of the Veronals, in 



