280 Prof. J. 1). Dana on some Results of 



School Laboratory, and thus differs but little from that of the 

 anhydrous dolerite of West Rock and the Palisades ; the pro- 

 portion is less instead of greater. There is hence a close simi- 

 larity to the other dolerite, notwithstanding the presence of 

 water. 



Since the dikes of this hydrated dolerite occur as parts of the 

 same series as the others and in the same regions, and are of the 

 same epoch and system of ejections, and moreover are exceptional 

 instead of the prevailing kind, it is apparent that the hydrous 

 character is owing to some exceptional condition attending or 

 subsequent to the eruption. The source of liquid rock was all 

 one; and the material must therefore have been essentially one 

 in kind; it could not have been locally hydrous. Portions of 

 the erupted viscid material might have encountered water on 

 the way to the surface and thus have become penetrated with 

 it*; so that the viscid material at the high temperature became 

 altered in part — some of its augite to chlorite, and perhaps some 

 of its felspar, with the lime of the augite, to zeolites, &c. Or 

 else this kind of change may have taken place through infiltra- 

 ting waters. The fact that the vesicular trap (now amygdaloidal) 

 is that which is hydrated renders it rather probable that while 

 zeolites and calcite may have been later made within the rock 

 by superfical action or infiltrating waters, the chief change was 

 contemporaneous with the eruption ; for the vesicular character 

 was undoubtedly then produced, and it was probably dependent 

 on the same moisture that, penetrating the rock, caused the rae- 

 tamorphism or hydration through the mass. Bunsen, in his 

 well-known paper on the igneous rocks of Iceland t, gives various 



* On this taking-in of moisture by liquid rock, Mallet, speaking of the 

 lava in the conduit of a crater, makes the following remarks : — " Within a 

 few years it has been proved that capillary infiltration goes on in all porous 

 rocks to enormous depths, and that the capillary passages in such media, 

 though giving free vent to water (and the more as the water is warmer), 

 are, when once filled with liquid, proof against the return through them of 

 gases or vapours. So that the deeply seated walls of the ducts leadiDg to 

 the crater, if of such material, may be red-hot and yet continue to pass 

 water from every pore (like the walls of a well in chalk), which is flushed 

 off into steam that cannot return by the way the water came down, and 

 must reach the surface again, if at all, by the duct and crater, overcoming 

 in its way whatever obstructions they may be filled with. And this re- 

 markable property of capillarity sufficiently shows how the lava (fused 

 below or even at or above the level of infiltration) may become interpene- 

 trated throughout its mass by steam-bubbles, as it usually, but not invari- 

 ably, is found to be." 



t Taylor's Scientific Memoirs, New Series, Part I. p. 33. The secon- 

 dary origin of the zeolites and chlorite found to penetrate some dolerite and 

 other kinds of igneous rock, as well as filling the cavities within them, I 

 sustained in an article in vol. xlix. of the first series of Silliman's Journal 

 (1845) ; and the various facts I have since observed have tended to confirm 



