THE INTRUSIVE DOLERITES AND ALLIED ROCKS 259 



Among the hundreds of dolerite sheets that have been 

 examined, none has been found to have the characters 

 of a lava-flow ; on the other hand, there is usually con- 

 clusive evidence in the hardening of the overlying rocks, 

 and in the sheet breaking through to a higher or lower 

 horizon, that the rock is intrusive, i.e., that it reached 

 its present position in a molten state after the surround- 

 ing sedimentary rocks had been deposited. 



The commonest modes of occurrence of the dolerites 

 are dykes and sheets or sills. The former are extremely 

 abundant in the Colony and are remarkably uniform in 

 width when their le-ngths are considered. The majority 

 extend in more or less straight lines for many miles ; 

 sometimes they form curved or sinuous outcrops. In 

 the Eastern Province a large proportion of the dykes 

 trend either north-east or north-west. 



Dolerite sheets are more noticeable in the Colony 

 than any other form of intrusion ; they are commonly 

 connected with dykes which in some cases may be re- 

 garded as the channels through which the rock com- 

 posing the sheets flowed. 



In the Transkei there are dome-like masses of 

 dolerite which resemble in their habit the intrusions 

 known as laccolites. The arching up of the overlying 

 beds characteristic of laccolites has not been observed, 

 and it is probable that many of these apparent laccolites 

 may really be curved sheets of dolerite. 



It was stated in the Introduction that the dolerite 

 intrusions are practically limited to that part of the 

 Colony which was not seriously affected by the earth- 

 movements that took place subsequently to the deposi- 



17* 



