6 /. IV. POWELL 



thus produced. From these sources the sedimentary rocks are 

 brought to be mingled with the eruptive rocks and intercalated 

 among them, while in turn they are thrust between the sediment- 

 ary rocks. 



Layers of rock of sedimentary origin appearing as strata 

 are commingled with other masses of rock of volcanic origin 

 which come from the interior. Sometimes the lava flows under 

 or between the sedimentary strata. When great masses of lava 

 are found in these conditions they are called lacolites. Thinner 

 sheets are called intrusive rock. Beds poured over the surface 

 are called coulees. The floods of lava come through fissures 

 and fill them both below and above coulees, intrusions, and 

 lacolites. Such fissure formations are called dikes; where the 

 lava comes forth in volcanoes, the orifices are filled with molten 

 rock which consolidates and are then called chimneys ; great 

 bodies of lava are ejected by some volcanoes as scoria and ashes, 

 and often the ashes are minutely comminuted ; the expulsion 

 of such material is doubtless due to the production of gases 

 and vapors, especially of steam, and the comminution is prob- 

 ably due to the explosive actions of particles of water expanded 

 into steam. Great volcanic cones are often formed by the piles 

 of scoria and ashes which are extravasated, and the ashes them- 

 selves when highly comminuted are drifted by the wind, some- 

 times far away from the locus of eruption. Beds of ashes and 

 scoria formed in this manner are called tuff. So the bodies of 

 rock formed by eruption are commingled with the bodies formed 

 by sedimentation, and all are known as formations. Both the 

 sedimentaries and the eruptives undergo a further change, which 

 to a greater or less extent obscures their origin, for the original 

 formations are metamorphosed, that is, recrystallized and lithi- 

 fied ; so that the planes of sedimentation are partly or largely 

 obscured and the beds of lacolites, intrusive sheets, coulees, 

 dikes, chimneys, and tuffs have a new structure imposed upon 

 them, and are then known as metamorphic rocks. 



An attempt has been made to define formations ; now they 

 must be considered in a new light. 



