NATURE AND ORIGIN OF VOLCANIC ROCKS. 41 



Most basalts were lava streams. The rocks are now often columnar, 

 or have a spheroidal or tabular structure. They are sometimes full 

 of natural air cavities, and are then said to be vesicular or scoriaceous. 

 When these cavities are infiltrated with minerals, the rocks are 

 termed amygdaloids. Behind Tobermory in Mull, these white infil- 

 trations are so numerous that the rock looks as though splashed over 

 with whitewash. The infiltrated minerals, which may form two-thirds 

 of the rock, are collectively termed zeolites. They nave been formed 

 by the water which percolated through the rock dissolving the felspars, 

 &c., and redepositing them chemically combined with water. Easaltic 

 rocks may contain large crystals ; they also form volcanic ashes. There 

 are many minor varieties of rock chiefly named from the zeolites which 

 enter into their composition. 



The preceding tables of the igneous rocks will be sufficient to 

 enable us to estimate the part which they take in forming the earth's 

 surface, though they give necessarily but an imperfect idea of their 

 varieties. Any classification into which they may be grouped must 

 always be a matter of convenience rather than an expression of the 

 necessary combinations of minerals into rocks, if we believe that the 

 materials of igneous rocks were originally the materials of stratified 

 formations. For the sorting power of water gives a differing mineral 

 composition to almost every mile of a formation as it recedes from 

 shore. If the whole deposit on the sea-bed were afterwards melted 

 up and ejected as lava, it would result that the parts near to land 

 would be rich in silica, and would contain the minerals in which 

 silica abounds, forming the quartz-bearing or so-called acidic rocks ; 

 while the parts more distant from land, such as certain clays, would 

 contain the minerals in which silica is deficient, and form the so-called 

 basic rocks. It may be artificial to classify either by the quartz or 

 the felspar in igneous rocks, but this is a chemical classification also. 

 At present we are unable to discover how those strata were spread 

 and composed, out of which igneous rocks have been reconstructed. 

 Obviously there must be many gradations of mineral character in the 

 igneous rocks which cannot be detected, on account of the fragmentary 

 way in which they burst through the earth's surface or become exposed 

 by the removal of superincumbent rock ; and these transitions would 

 probably be as complete were they known, as are the gradations of 

 texture in the rocks which result from differences in the conditions 

 under which they cooled ; which permit the same original rock sub- 

 stance to become a mass of large crystals, finely crystalline, compact 

 like biscuit china, glassy, vesicular, scoriaceous, or ashy ; or when 

 vesicular to be so altered by infiltration that its whole character is 

 changed. We must bear in mind that while the liquefaction of sand- 

 stones would give certain granitic rocks, and liquefaction of clays 

 would yield the greenstones, these materials may often be so contorted 

 and folded together with limestone, that the igneous rock resulting 

 from their fusion would be intermediate in character, or combine 

 minerals which are usually limited to different groups of rocks. 



