IGNEOUS ERUPTIONS. 



717 



not ragged like those of lavas, 

 pressure when formed 



a fact due to their having been under 



When cellular, the rock is said to be amygda- 

 Fig. 1116. 





Basaltic columns, coast of Illawarra, New South Wales. 



loiclal ; it is often called an amygdaloid (from amygdalum, an almond), 

 in allusion to the fact that the cellules or little cavities are filled 

 through subsequent infiltration, and the filling, like the cavity, is often 

 almond-shaped. 



The manner of filling these cavities, and the nature of the materials, 

 are explained on page 784. Amygdaloidal varieties of dolerytic rocks 

 usually contain considerable moisture, and often also disseminated 

 chlorite. They thus show that they were subjected to a free supply 

 of moisture, from a subterranean source, when in process of eruption ; 

 and this fact accounts for the existence of the cellules. 



These igneous rocks sometimes form layers, interstratified with ordi- 

 nary sandstones or other sedimentary rock, and even uncompacted 

 sand and gravel ; they having flowed out over a region, it may be for 

 hundreds of miles, covering up the strata previously laid down, and 

 then becoming the basis for new deposits of sand or mud. They thus 

 lie between beds, in all the geological formations. Examples of Ameri- 

 can Lower Silurian rocks of the kind are described on page 185. In 

 the British Lower Silurian, in Wales, they occur among both the Llan- 

 deilb and Bala formations. The Triassic or Jurassic trap, on the At- 

 lantic Border of North America, affords another example, as described 

 on page 418 ; but the beds here have come up through sandstone rocks, 

 without extensive overflows. The Cretaceous era, and still more the 

 Tertiary and Quaternary, were remarkable for the extent of the erup- 

 tions over the western slope of the Rocky Mountains (p. 524), and 

 also in Britain (p. 525) and many parts of Europe, and on other conti- 

 Hents. 



The following sketch, from Hayden's Report for 1873, repre- 



