274 PETROGRAPHY OF THE INTRUSIONS 



As a rule the dolerites have a well-developed ophitic 

 structure ; the plagioclase crystals are to a greater or 

 lesser extent enclosed by the augite, but near the edges 

 of the intrusions the augite is granular or forms rather 

 imperfect crystals. 



In thin dykes and sheets the structure is distinctly 

 porphyritic, crystals of olivine, augite and plagioclase 

 being embedded in a fine-grained matrix of augite grains 

 and very small plagioclase crystals, often with a con- 

 siderable amount of brown glass. Occasionally an al- 

 most pure glass, tachylite, is found at the contact of a 

 sheet or dyke with the surrounding rocks, or in the 

 form of thin veins traversing the dolerite or more rarely 

 the sedimentary rocks. Tachylite is a black substance 

 with a glassy appearance ; it looks not unlike bright 

 bituminous coal, for which it has often been mistaken in 

 this country. The greater specific gravity and hardness 

 of the tachylite, however, distinguish it at once from coal, 

 and of course it will not burn. Porphyritic crystals of 

 augite and plagioclase may occur in the tachylite, and 

 the glass is sometimes converted into an opaque stony 

 material along joints. 



Both the tachylite and the glassy dykes and sheets 

 owe their peculiarities to rapid cooling. The thick sheets 

 of dolerite naturally took a longer time in cooling than 

 the smaller bodies of molten rock, and consequently the 

 minerals were able to develop more thoroughly in them 

 than in the latter, so the rock as we see it now is 

 coarsely crystalline in the one case and finely crystal- 

 line or glassy in the other. The fact that the well- 

 formed crystals of olivine are often abundant in the 



