CHAPTER XII. 



ORIGINAL CHARACTERS AND LITHOGENESIS OF THE EXOGENETIC 

 ROCKS. THE PYROCLASTICS AND THE AUTOCLASTICS. 



The original structures of Exogenetic or clastic rocks are mainly 

 those formed during their deposition. Therefore the structures of 

 each division of these rocks will show individual peculiarities, and 

 hence each division will be considered by itself. 



PYROCLASTIC ROCKS. 



Modern Pyroclastics. Tuffs and volcanic breccias (pyrolu- 

 tytes, pyrarenytes, and pyrorudytes) are typically formed about the 

 volcanic vents which have furnished the material. (Their essential 

 lithic characteristics have been described in Chapter VI.) Ordi- 

 narily the distance from the volcano at which the deposit will form 

 will be dependent on the fineness of grain. Thus pyrorudytes will 

 be formed close to the volcano, while the pyrolutytes will often 

 form at great distances from the vent. These latter are often car- 

 ried very far by the winds, and finally deposited as ?eolian or wind- 

 blown volcanic ash (anemopyrolutyte). In such cases the structure 

 of the deposit partakes of that of other anemoclastic rocks. Many 

 tufifs are deposited in water, and so become closely allied to typical 

 hydroclastic rocks and partake of their structure. The finer water- 

 laid volcanic tuffs (Hydropyrolutytes) often show a perfect stratifi- 

 cation, and they sometimes are rich in organic remains. An ex- 

 ample of finely bedded pyrolutytes is found in the fine paper shales 

 of volcanic ashes of Florissant, Colorado, where a wealth of beauti- 

 fully preserved insects, leaves, and silicified trees testifies to the 

 adaptability of these deposits to the preservation of fossils. These 

 deposits are commonly referred to a Tertiary lake caused by the 

 damming of a valley by a lava flow. The possibility that the de- 

 posits are those of a river flood plain or playa lake must not be 

 overlooked. The great abundance of insects, spiders, and leaves, 

 the erect tree trunks, and the scarcity of aquatic animals point this 



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