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OUTLINES OK FIELD-GEOLOGY 



PART f 



study carefully a good series of specimens of various tuffs 

 from different geological formations, and learn to distin- 

 guish them from mere mechanical detritus derived from 

 the superficial disintegration of igneous rocks. 



But the proof of former volcanic action is vastly 

 strengthened when we find not only the consoli< 

 ashes, but also the lava-streams of the period (/, /'. 

 64). A truly interstratified sheet of crystalline rock is, 

 in fact, a lava-stream which has been poured out at the 

 surface, either on land or under water, and shows the 

 distinctive characters of such a bed. Thus it is commonly 



FIG. 66. Upper surface of an interstratified igneous sheet with sedimentary 

 strata lying upon it. 



rough and slag-like towards its top (Fig. 66) and bottom, 

 and most compact about the centre. The strata lying 

 upon it, having been deposited there after the emission 

 of the lava, are not altered, have no portions of their 

 substance entangled in the crystalline rock, but, on the 

 contrary, may contain detached fragments of the latter. 



It is eminently characteristic of lava to acquire a cell- 

 ular texture, from the expansion of the abundant steam 

 imprisoned within it at the time of eruption. This feature 

 is specially developed at the top where, the pressure being 

 least, the vapour has had most freedom of motion. As 

 the vesicles appeared while the rock continued to move, 



