108 



LITHOLOG1CAL GEOLOGY. 



which forms the Giants' Causeway may in other places be interstrati- 

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



Fig. 115. 





Basaltic columns, coast of Hlawarra, New South Wales. 



wherever it takes place, may be followed afterward by deposits of 

 sand or other sediment. 



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 ordinary mode of 

 action by water lays down sediments in layers. But these accumu- 

 lations — 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 fragmental, 

 and stratified and unstratified igneous rocks ; and, from the obliteration 

 of the planes of deposition by metamorphism, there may be unstratified 

 metamorphic rocks, like granyte, as well as stratified. 



On the subject of the structure of these rocks, it is only necessary 

 to refer to the ordinary massive structure of granyte and trachyte, etc., 

 and to the columnar structure met with among igneous rocks. The 

 last is represented in the figure given above. There are all shades 

 of perfection in this columnar structure, from prisms of great height 

 with perfectly plane sides, to a mere tendency to prismatic forms ; and 

 also from this less perfect prismatic character, to the massive structure 

 with no trace of columnar fracture. 



For a continuation of this subject, see the chapter on igneous 

 operations, under Dynamical Geology. 



( 1 .) General nature of veins. — The vein condition. — Veins are 

 narrow plates of rock intersecting other rocks. They are the fillings 

 of cracks or fissures ; and, as these cracks or fissures may either 

 extend through the earth's crust and divide it for long distances, or 



