0. Pouktt Scrope — On Lavas. 101 



vestigation — namely, through fcho known to the unknown — to the 

 necessity of acquiring some more definite notions than can ha said to 

 prevail at present, on the character and mode of production of the 

 hypogene rocks which are exposed at or near the surface, before 

 proceeding to speculate further on the condition of those that pos- 

 sibly exist at great depths beneath it. 



Among the facts relating to Volcanic Geology which have not as 

 yet, in my opinion, been satisfactorily determined by observation, 

 although freely open to it, I may mention, as especially deserving of 

 examination, the character and composition of lavas, as they exude 

 from the vents or fissures opened for their issue through the solid 

 crust. In the year 1856 I read before the Geological Society of 

 London a paper on this subject (as well as on the mode of formation 

 of Volcanic Cones and Craters), which does not appear to have 

 atti'acted much attention from Geologists, but which I think con- 

 tained suggestions worthy of their consideration. (See Quart. Journ. 

 Geol. Soc, vol. xii., p. 338.) If, indeed, we wish to form any 

 reliable opinion upon the nature and character of the subterranean 

 matter underlying the sedimentary rocks, surely a close examination 

 of those portions of this matter which are in so many places rising 

 to the surface and spreading themselves over it, is indispensable. 



In the paper above referred to I showed how little foundation 

 there is for the generally received opinion that all lavas, as they 

 issue from a volcanic vent, are in a state of complete fusion, like 

 melted glass or metal, and that it is only through a process of slow 

 cooling they afterwards assume a crystalline texture. I pointed out 

 that there is good reason on the contrary for believing that the greater 

 number, if not all, of the crystals observable in lava after cooling and 

 hardening, (and the microscope discloses them where they are not 

 visible to the naked eye), existed there in a more or less complete 

 form previously to its emission. Some of the facts mentioned in 

 support of this view I may briefly recapitulate. Such are, the ex- 

 tremely stiff and tenacious character of most lavas, as they issue at 

 a white heat from the volcanic orifice, making it difficult to thrust a 

 pointed iron rod into them; the absence of a vitreous texture, even in 

 their superficial portions, or in the scoriEe, torn seething hot from the 

 surface of the liquid matter within the vent by gaseous explosions ; 

 the instantaneous consolidation of exposed surfaces in cellular or 

 porous slabs or cakes, which on fracture are found to have the same 

 crystalline texture as the interior of the current; the cracked and 

 more or less vitrified aspect of the felspar crystals of many trachytes ; 

 the broken and dislocated appearance of the leucites, felspars, and 

 other crystals in many basalts ; the frequent arrangement of the 

 longest axes of such crystals in the direction of the rock, that is, of 

 the movement of the lava when liquid ; the finer grain often exhi- 

 bited towards the tail or extremity of a current than at its source, as 

 if the crystals had been broken up by friction as the matter moved 

 on ; the brecciated lavas which appear to have enveloped numerous- 

 fragments of the same equally crystalline material without any 

 fusion even of their finest angles, etc. 



