Lava 47 



Large crystals are, it is true, very often found in lavas, but these 

 were formed before the ejection of the mass from the volcano. 

 Such crystals frequently contain enclosures of glass, which indi- 

 cate that the crystallization went on while the surrounding mass 

 was still fluid. The edges and angles of these crystals are often 

 corroded by the action of the melted portion of the lava, and the 

 motion of the stream often cracks them. These facts go to prove 

 that the large crystals were complete when the lava, as a whole, 

 was still fluid and in motion. Stromboli ejects great numbers of 

 perfect crystals of augite, which must have existed in the molten 

 lava of the vent. The lavas which contain large crystals embedded 

 in a fine stony or glassy base are said to be of a porphyritic 

 texture. 



It is important to remember that all these various textures may 

 be found in one continuous rock mass, and bear witness as to the 

 circumstances under which each part cooled and solidified. These 

 textures also recur again and again in ancient rocks and enable us 

 to determine their volcanic origin. The processes of rock destruc- 

 tion have in many cases laid bare deep-seated masses which were 

 plainly once melted like true lavas, but which have cooled very 

 slowly and under great pressures. In such rocks the texture is 

 usually coarsely crystalline and shows no traces of glass or scoriae. 

 Between the surface lava flows and such deep-seated reservoirs 

 every form of transition may be traced, often in continuous rock 

 masses. 



Where several successive lava flows issue from one vent, at in- 

 tervals which allow one stream to be consolidated before the next 

 is poured out over it, a rough bedding or stratification results, 

 each flow being perfectly distinguishable when seen in section. 

 Deceptive resemblances to the true stratification of sedimentary 

 rocks (see p. 145) may thus arise, especially when the exposed 

 section is short. But the wedge-like form of the sheets, the absence 

 of bedding within the limits of each flow, and the nature of the 

 rock itself, always enable us to distinguish these masses from the 

 sediments which have been stratified by the sorting power of 

 water. 



