660 



VOLCANIC ROCKS OF 



[Ch. XXX 



ing (fig. 712), the overlying clay has been greatly altered and hard- 

 ened by the igneous rock, and occasionally contorted in the most 

 extraordinary manner ; yet the lamination has not been obliterated, 

 but, on the contrary, rendered much more conspicuous, by the indu- 

 rating process. 



In the foregoing woodcut (fig. 713) I have represented a portion 

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

 laminse 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, and 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 superincumbent ter- 

 tiary strata. In one place the lava ramifies and terminates in thin 

 veins, from a few feet to a few inches in thickness (see fig. 714). 



Fig. 714. 



b a 5 c a & 



Clay. Lava. Clay. Altered. Lava, Clay, &c. 

 Newer Pliocene strata invaded by lava, Isle of Cyclops (horizontal section). 

 a. Lava. &. Laminated clay and sand. c. The same altered. 



The arenaceous laminas are much hardened at the point of contact, 

 and the clays are converted into siliceous schist. In this island the 

 altered rocks assume a honeycomb structure on their weathered surface, 

 singularly 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 analcime, 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, whether we sup- 

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

 rents of the contiguous sedimentary strata. 



