PYROCLASTIC ROCKS 203 



APPENDIX 



THE PYROCLASTIC ROCKS 



These rocks are formed out of the fragmental materials ejected 

 from volcanoes, The materials are of course igneous, but the 

 rocks themselves differ from the typical igneous rocks in several 

 important respects. They have not been formed in their present 

 state of aggregation by cooling from a molten mass, and in many 

 cases they are more or less distinctly stratified. It seems best, 

 therefore, to group them separately, under the name pyroclastic. 



Volcanic Agglomerate or Breccia is a mass of angular blocks of 

 lava, with which may be mingled fragments of sedimentary rocks, 

 which the volcano has torn off from the sides of its chimney. The 

 blocks may be loose or cemented together into hard rock by a 

 filling of finer materials. Ordinarily the breccia is formed only 

 near the vent, but sometimes it is developed on a great scale, as 

 in the eastern part of the Yellowstone Park. 



Tuffs are masses of volcanic ashes and dust, which accumulate 

 in beds, either on the land or in bodies of water. Even in falling 

 through the air, the particles are sorted, in some degree, in 

 accordance with their size, and the tuffs are thus usually stratified, 

 and sometimes have fossils in them. When accumulated under 

 water, the ashes are, of course, stratified and may be mingled with 

 more or less sedimentary debris. Such subaqueous tuffs pass into 

 the ordinary sedimentary rocks, by the gradual diminution of the 

 volcanic material. When examined under the microscope, even 

 the finest tuffs are found to consist of crystals and particles of glass. 



The volcanic breccias and tuffs may best be classified in accord- 

 ance with the nature of the component fragments. Thus, we find 

 rhyolite tuffs and breccias, andesite tuffs and breccias, basaltic 

 tuffs and breccias, and the like. 



