136 



latter in the earth movements that have affected the districts in which they 

 occur. Corresponding rocks are found in this country in Devon and 

 Cornwall. 



Dolerite. This name was applied by HAUY to a rock essentially composed 

 of augite and plagioclase. The type rock from Meissner contains also a little 

 olivine. ROSENBUSCH uses the term as subordinate to basalt, and merely 

 employs it for a coarse grained variety of the latter rock. It is, therefore, 

 according to ROSENBUSCH a post-cretaceous rock composed essentially of 

 plagioclase, augite and olivine. Mr. ALLPORT (1) has proposed that dolerite 

 should be raised to the rank of a group name ; that it should be used without 

 reference to geological age ; and that it should be employed for all basic 

 igneous rocks essentially composed of plagioclase and augite, and possessing a 

 texture intermediate between gabbro and basalt. From this point of view 

 olivine is not regarded as an essential constituent. In this work we shall 

 follow Mr. ALLPORT. The term dolerite will be applied to a medium grained 

 plagioclase-augite rock of basic composition. The different varieties depending 

 on the presence of certain accessory constituents will be termed olivine-dolerite, 

 enstatite-dolerite, mica- dolerite, hornblende-dolerite, &c. 



Anamesite. This term was introduced by LEONHARD for the finer grained 

 but distinctly crystalline members of the dolerite group. 



Basalt. This is a very old term first used by STRABO and PLINY for 

 certain black rocks from Egypt, which were employed in the arts in very early 

 times. CoRDIER proved in 1815 by the microscopic examination of the 

 powder of the compact rocks to which the term had been applied in Europe, 

 that these rocks resembled dolerite in mineralogical composition. In 1870 

 ZIRKEL (2) separated the continental basalts into three main groups felspar- 

 basalts, nepheline-basalts and leucite-basalts. In 1877 ROSENBUSCH proposed 

 to make olivine an essential constituent of basalt, and defined basalt as a 

 post-cretaceous rock essentially composed of plagioclase, leucite or nepheline 

 with augite and olivine. Basalt is used by ROSENBUSCH as the group name, 

 and dolerite is applied to a coarsely crystalline basalt. As in this country the 

 distinctly crystalline varieties prevail over the compact forms, we take dolerite 

 as the group name. 



Greenstone. This is an old term which has been applied to many of 

 the basic igneous rocks associated with palaeozoic and especially with 

 lower palaeozoic sediments. The green colour is due either to chlorite or 

 hornblende. The word has done good service in the past as a field term and, 

 where it is impossible to give a precise mineralogical description, is not 

 without its use at the present day. Many of the old greenstones are dolerites 

 altered by surface agencies (diabases), or by contact or regional metamorphism 

 (epidiorites and proterobases). Diabases abound in Wales and epidiorites and 

 proterobases in Devon and Cornwall. 



Melaphyrc. This term was introduced by A. BROGNIART for rocks 

 consisting of a "pate noire d'amphibole petrosiliceux envellopant des crystaux 



(1) Q.J.G.S., vol. XXX., p. 529. 



(2) Uber die Mikroskropische Zusammensetzung und Structur der Basaltgesteine. 

 Bonn. 1870. 



