.62 



OUTLINES OF FIELD-GEOLOGY 



PART I 



faces ; in a dyke or vein they vary according to the 

 inclination of the mass, being horizontal when the dyke 



iral (Kig. 55). 



When an igneous rock has been cut through by a fault, 

 the fractured surface, especially when rough and shattered, 



Fie. 56. Section of intrusive sheet or sill (j), underlying strata (*), and 

 overlying the group (a), and connected with a dyke or pipe (tf). 



may assume a deceptive resemblance to the original wall 

 of a truly intrusive mass. But in such a case it would 

 probably be found that the surfaces both of the igneous 

 rock and of those next it showed, in their striated and 

 polished appearance, evidence of having been made to 

 grind against each other as solid masses (slickensides). 



Dykes vary in thickness from less than an inch to 

 100 feet or more, and from a vertical to a more or less 



Fie. 57. Upper surface of an intrusive igneous sheet or sill with overlying shale. 



inclined position. They often rise along previous lines 

 of fracture ; but in the great majority of cases the basalt- 

 dykes of Tertiary date, so abundant in Britain, have filled 

 fissures without any perceptible vertical throw of the 

 rocks on either side. Sometimes a dyke may be found, as 



