IS OUT I I Ms OF FIELD-GEOLo PARTI 



with all the once melted rocks like the lavas ; and (2) the 

 Fragmental (Pyroclastic), including the consolidated vol- 

 canic ashes, tuffs, and conglomerates. As a rule, these 

 two series are broadly and distinctly marked off from 

 each other, both by their lithological characters and by 

 their behaviour as rock-masses. 



The Crystalline igneous rocks, as their name indi< 

 have solidified from molten or from aquo-igneous solu- 

 tions; sometimes remaining still in the condition of 

 sometimes completely crystalline, and with every possible 

 gradation between these two extremes. But though for 

 the most part recognisably crystalline in the field, they 

 are not always so. Many ancient eruptive rocks, for 

 example, might in hand-specimens be taken for pieces of 

 hardened clay. In such cases, should the lithological 

 characters be indefinite, the true character of the rock 

 may usually be ascertained from its relation to the sur- 

 rounding masses. If these are obscured, the final appeal 

 may be to the microscope. Hence the learner must be 

 prepared for endless varieties of texture, colour, hardness 

 and softness, toughness and friableness, among the once 

 molten igneous rocks. 



The Fragmental series is less varied, as its members 

 consist of fragmentary materials, derived partly from the 

 explosion of lava rising in the throats of volcanoes, and 

 partly from the debris torn from the sides of the volcanic 

 funnels and craters. These rocks are essentially charac- 

 terised by the fragmental nature of their component 

 particles, which may vary in texture from the finest im- 

 palpable dust up to blocks weighing many tons. Con- 

 siderable variety must obviously exist in the coarseness 



