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actinolitic. Less frequently it occurs in the form of irregular crystalline 

 grains which are not fibrous. It will then be spoken of as compact horn- 

 blende. It varies somewhat in tint, but is almost always some shade of 

 green. One variety, commonly occurring in the form of actinolite, shows 

 the following pleochroism : a, very pale green ; ft, green ; 7, rich bluish 

 green or greenish blue. This is especially found near the granite masses. 

 A pale brown, fibrous hornblende (brown uralite) is sometimes present. 

 For purposes of distinction we shall speak of the rocks containing original 

 hornblende as hornblende-dolerites or -diabases, and limit the terms 

 proterobase and epidiorite to those with secondary hornblende. Thus the 

 original rocks may be roughly divided into two groups. 



(1) Dolerite or diabase proper. 



(2) Hornblende-dolerite or -diabase. 



A deep brown biotite is often associated with the basaltic hornblende 

 and sometimes occurs to the exclusion of the latter. The varieties thus 

 produced may be designated hornblende-biotite-dolerite and biotite-dolerite. 



Turning now to the rocks with secondary hornblende, and leaving out 

 of account those which show more or less foliation, we note every gradation 

 from a rock essentially composed of plagioclase, augite and iron-ore, to one 

 formed essentially of plagioclase, hornblende and iron-ore. To the latter 

 variety we shall apply GUMBEL'S term, epidiorite. The term proterobase 

 will be applied to rocks in which more or less original augite remains. 

 According to this nomenclature uralite-diabase becomes a variety of pro- 

 terobase. It must not be supposed that there is any sharp line between 

 dolerite (diabase), proterobase and epidiorite. One and the same rock-mass 

 may furnish all three varieties. The terms proterobase and epidiorite are 

 merely used for the purpose of shortening descriptions. 



The dolerites almost invariably contain a certain amount of chlorite 

 and other alteration products. They may therefore be spoken of as 

 diabases. They frequently exhibit the ophitic structure in the greatest 

 perfection and relics of this structure are often preserved in the pro- 

 terobases and epidiorites. When this is the case the adjective " ophitic " 

 will be used in speaking of these rocks. This preservation of the 

 ophitic structure in many of the epidiorites is of the utmost importance 

 because ophitic epidiorites frequently pass by the most insensible grada- 

 tions into foliated rocks the origin of which would otherwise remain 

 a mystery. 



Having explained the nomenclature which we shall adopt we proceed 

 to consider the rocks in greater detail. The least altered rocks occur in 

 the eastern portion of the district ; that is in Devon and east Cornwall. 



Typical ophitic diabases have been observed at Yealmpton, Rock, 

 Park House near Dartington, Pollaphant, Catacleuse near Padstow. 

 South of Anstie's Cove near Torquay and at South Pretherwin. The 

 felspars are always more or less turbid in consequence of alteration. 

 They occur in forms giving lath-shaped sections. The augite is pale 

 in colour and occurs in large ophitic masses. It is often seen to 

 have been more or less replaced by green chloritic minerals in which 



