PART II. CHAPTER XXIII. 



277 



Volcanic Rocks of the Cretaceous Period, 



Miocene period are discovered. After the filling up or drainage 

 of the ancient lakes, huge piles of trachytic and basaltic rocks, 

 with volcanic breccias and conglomerates, accumulated to a 

 thickness of several thousand feet, and were superimposed upon 

 granite, or the contiguous lacustrine strata. The greater portion 

 of these igneous rocks appear to have originated during the Mio- 

 cene period, and extinct quadrupeds of that era, belonging to the 

 genera Mastodon, Rhinoceros, and others, were buried in ashes 

 and beds of alluvial sand and gravel, which owe their preserva- 

 tion to sheets of lava which spread over them. 



Ci'etaceous period. — Although we have no proof of volcanic 

 rocks erupted in England during the deposition of the chalk and 

 green-sand, it must not be supposed that no theatres of igneous 

 action existed in the cretaceous period. M. Virlet, in his account 

 of the geology of the Morea, (p. 205.) has clearly shown that 

 certain traps in Greece, called by him ophiolites, are of this date; 

 as those, for example, which alternate conformably with creta- 

 ceous limestone and green-sand between Kastri and Damala in 

 the Morea. They consist in great part of diallage rocks and 

 serpentine, and of an amygdaloid with calcareous kernels, and a 

 base of serpentine. 



In certain parts of the Morea, the age of these volcanic rocks 

 is established by the following proofs : first, the lithographic 

 limestones (see p. 199.) of the cretaceous era are cut through by 

 trap, and then a conglomerate occurs at Nauplia and other places, 

 containing in its calcareous cement many well-known fossils of 

 the chalk and green-sand, together with pebbles formed of rolled 

 pieces of the same ophiolite, which appear in the dikes above 

 alluded to. 



It was before stated that at Tercis, near Dax, in the depart- 

 ment of the Landes, in the south of France, highly inclined 



Fig. 287. Adour R. Luy R. Puy Arzet. 



Chalk and volcanic tuff in the environs of Dax. 

 E Inclined beds of chalk, and conformable volcanic tuff 

 a. b. c. d. Gravel, sand, and tertiary strata. 



strata of limestone and marl occur, containing the fossils of the 

 chalk, the inclined strata being in great part concealed by 

 unconformable tertiary formations. In one section in this dis- 

 trict I observed, alternating with thin layers of volcanic tuff, 

 vertical cretaceous beds, which are perfectly conformable. Such 



