524 



VOLCANIC ROCKS OF 



[Ch. XXX 



Kg. 6G1. 



These strata dip to tlie N. W., and rest on a mass of columnar lava (see 

 fig. 660), in which the tops of the pillars are weathered, and so rounded 

 as to be often hemispherical. In some places in the adjoining and largest 

 islet of the group, which lies to the north-eastward of that represented 

 in the drawing (Qg. 660), the overljing clay has been greatly altered, 

 and hardened by the igneous rock, and occasionally contorted in the 

 most extraordinary manner; yet the lamination has not been obliterated, 

 but, on the contrar}^, rendered much more conspicuous, by the indurat- 

 ing process. 



In the annexed woodcut (fig. 661) I have represented a portion of 

 the altered rock, a few feet square, where the alternating thin laminaB 



of sand and clay have put on 

 the appearance which we often 

 observe in some of the most 

 contorted of the metamorphic 

 schists. 



A great fissure, running from 

 east to west, nearly divides this 

 larger island into two parts^ ,9nd 

 lays open its internal structure. 

 In the section thus exhibited, a 

 dike of lava is seen, first cutting 

 through an older mass of lava, 

 and then penetrating the super- 

 incumbent tertiary strata. In 

 one place the lava ramifies and 

 terminates in thin veins, from a 

 few feet to a few inches in 

 thickness. (See fig. 662.) 



The arenaceous laminaa are 

 much hardened at the point of 

 contact, and the clays are con- 

 verted into siliceous schist. In 

 this island the altered rocks as- 

 sume a honeycombed structure 

 on their weathered surface, sin- 

 gularly contrasted with the smooth and even outline which the same 

 beds present in their usual soft and yielding state. 



The pores of the lava are sometimes coated, or entirely filled with 

 carbonate of lime, and with a zeolite resembling anal ci me, which has 

 been called cyclopite. The latter mineral has also been found in small 

 fissures traversing the altered marl, showing that the same cause which 

 introduced the minerals into the cavities of the lava, vzhether we sup- 

 pose sublimation or aqueous infiltration, conveyed it also into the open 

 rents of the contiguous sedimentary strata. 



Post- Pliocene formcdions near Naples. — I have traced in the '^Prin- 

 ciples of Geology" the history of the changes which the volcanic region 



Contortions of strata in the largest of the Cyclopiaa 

 Islands. 



