118 



LITHOLOGICAL GEOLOGY. 



marks of stratification. The sketch below represents a scene 

 among rocks of this kind in Australia. The dome-shaped masses of 



Fig. 115. 



Basaltic columns, coast of Illawana, New South Wales. 



trachyte in some regions of ancient volcanoes, and the interior mass 

 of many great volcanoes, — sometimes exposed to view through rend- 

 ings of the mountain or denudation by water, — are also examples. 

 But the ordinary outflows of liquid rock from volcanoes usually 

 produce layers, which are covered afterwards by others in succession ; 

 and volcanic mountains, therefore, have to a great extent a strati- 

 fied arrangement of the rock-material, and not less perfectly so 

 than bluffs of stratified limestone. Moreover, the same rock which 

 forms the Giants' Causeway may in other places be interstratified 

 among sandstones and shales ; for the layer of igneous outflow, 

 wherever it takes place, may be followed afterwards by deposits 

 of sand or other sediment. 



129. Another example of unstratified material is found in the 

 loose pebbles and stones which cover a large part of the northern 

 half of both the American and European continents. Any ordi- 

 nary mode of action by water lays down sediments in layers. But 

 these accumulations — often called drift — are of vast extent and 

 without layers. Wherever the same kind of material is in layers, 

 it is then said to be stratified; and thus it is distinguished from the 

 unstratified. 



There may, therefore, be both stratified and unstratified sedi- 

 ments, and stratified and unstratified igneous rocks ; and by the 

 obliteration of the planes of deposition by metamorphism there 

 may be unstratified metamorphic rocks like granite, as well as 

 stratified. 



130. On the subject of the structure of these rocks, it is only 

 necessary to refer to the ordinary massive structure of granite 



