242 



skeleton rhombohedra (ilmenite ?) as in the ophitic diabases of Devon and 

 East Cornwall. The rock described above appears to be in every respect 

 identical with the " Ottfjall-Diabas " of TORNEBOHM. (I) It differs from the 

 other ophitic olivine-dolerites known to us in this country in the character 

 of its olivines. 



In the majority of cases the dykes in the eastern portion of the 

 Lizard district must be described as epidiorites ; in other words they consist 

 essentially of plagioclase and secondary hornblende. The plagioclase may 

 be either turbid or water-clear and the hornblende may be uralitic, actino- 

 litic or compact. In a large number of cases the structure of the epidiorites 

 is distinctly ophitic ; the lath-shaped felspars retain their original outlines 

 and penetrate irregular masses of uralitic hornblende which correspond in 

 form to the original ophitic augite. Iron ores occur in the epidiorites 

 exactly in the same way as they do in the dolerites. The absence of olivine 

 from the epidiorites is a noteworthy feature. The secondary hornblende is 

 generally of a pale green colour ; pale brown varieties may, however, be 

 occasionally observed. That the epidiorite and dolerite dykes are of the 

 same age is proved by the similarity in their mode of occurrence and by 

 the fact that transitions may occasionally be observed from one rock to the 

 other in the same dyke. The uralitization or amphibolization of the augite 

 appears to be accompanied by the development of hornblende at the 

 expense of the olivine. In some of the olivine-dolerites the olivines are 

 seen to be surrounded by a narrow zone of a greenish mineral which 

 possesses the double-refraction of hornblende and may probably be referred 

 to that mineral. 



Foliation has not been extensively developed in the dykes under 

 consideration, but it occurs occasionally as, indeed, we have already 

 mentioned. It is usually of the flaser-type, but sometimes we find the 

 more even foliation characteristic of normal hornblende-schists. Sometimes 

 the shearing movement has been concentrated, as it were, along certain 

 planes which may be recognized in the thin sections ; at other times 

 it has been distributed over larger areas. Wherever it occurs the 

 original microscopic structure of the rock has of course been 

 more or less destroyed. 



The south-western portion of the Lizard district is a region of intense 

 mechanical metamorphism. The structural characters are similar in many 

 respects to those which may be obtained along the great line of disturbance 

 in the north-west of Scotland. (2) One type of rock very common in this 

 highly metamorphic region is a green actinolitic or hornblendic schist. 

 In association with this schist we find lenticles and bosses of epidiorite 

 often distinctly porphyritic and having the forms of the original porphyritic 

 crystals (felspars) well defined. These masses of rock, undoubtedly of 



(1) N. J. 1877, p. 273. 



(2) The resemblance between this district and the north-west of Scotland has been extended 

 since this was written by MB. Fox's discovery of strong hornblendic gneisses with associated 

 basic igneous rocks in the Islands off the Lizard Coast. 



