The Transkci Gap. 75 



which sphene is very often associated, and iron pyrites. Apatite is 

 always present, sometimes in considerable quantity. 



The rock forming the dykes of the Gap may be called a quartz- 

 mica-augite diorite ; it differs from the olivine-dolerite very con- 

 siderably in the absence of olivine, and in the presence of large 

 amounts of hornblende, brown mica and quartz, as well as of the 

 more acid varieties of plagioclase. It is very noticeable, however, 

 that none of the minerals which characterise the Gap-rock are 

 foreign to the olivine dolerites, and in the case of Kologha sheet the 

 dolerite in parts approaches the Gap-rock in character rather closely, 

 by the increase in the amount of hornblende, red mica, and the 

 zoning of the plagioclase. The affinity between the two rocks is 

 sufficient to make it preferable to regard the Gap-rock as derived 

 from the magma which supplied the dolerite intrusions rather than 

 the result of a quite different order of events. If we consider the 

 Gap-rock as a late product of the magma after the dolerite had been 

 got rid of, our view will explain the facts observed under the micro- 

 scope and in the field, for while the evidence of a microscopic 

 examination shows that the Gap-rock and the dolerite are genetically 

 related, the field evidence conclusively proves that the latter rock 

 had solidified before the former was intruded through it. 



