102 The Paragenetlc Relations of Minerals. 



is here evident in the course of from 50 to 200 years, and 

 Professor Breithaupt considers that this long-continued influ- 

 ence may have converted many rocks into substances quite 

 different from what they were at their original solidification. 



Generally speaking, fresh unaltered phonolite is not vesi- 

 cular, but only slightly porous, and scattered blocks of it are 

 then only superficially weathered ; vesicular phonolite, on the 

 contrary, is scarcely anywhere met with in a fresh state, but 

 is decomposed throughout. Struve shewed that the weathered 

 crust of phonolite had lost its potash and soda. E. G. Gmelhvs 

 investigation of phonolite shewed that it consisted of a zeo- 

 litic substance, soluble in acids, and a silicate having the 

 composition of felsite, insoluble in acids, and further, that in 

 the vesicles and veins the former was more or less wanting, 

 sometimes entirely so, while these vesicles and veins con- 

 tained natrolite, which cannot be regarded as of simultane- 

 ous formation with the phonolite, but has probably been 

 formed by the extraction of the rock by water. Walterhau- 

 sen states that the zeolites of Iceland generally occur in a 

 crumbled earthy bed of decomposed rock, and that the calcite 

 occurs there in the same manner. 



It is probable that the vesicular cavities in rocks have not 

 in all instances been produced at the original formation of 

 the rock. Pearlstone, pitch stone, and some f el sites, become 

 vesicular when heated. 



Cavities resembling vesicles have likewise sometimes been 

 formed by the decomposition and removal of imbedded no- 

 dules, — as for instance in basalt, — by the decomposition of 

 the olivine. 



The substances which are contained in these vesicular ca- 

 vities are generally of subsequent date, and have most pro- 

 bably been formed by an extraction of the rock. In some 

 agates, this mode of production is almost obviously indicated 

 by the structure. The vesicular cavities containing zeolites, 

 heavy spar, calcite, phrenite, and copper and quartz, differ 

 from those containing agate, in not presenting the point of 

 infiltration so characteristic of the latter. This would ap- 

 pear to indicate that the substances have been introduced in 

 a different manner in the two eases. Thev could not have 



