454 MR. A. HAEKEE ON THE ERUPTIVE ROCKS Ilf THE 



diabase from Bodowen, Anglesey, and in many of the diabases of 

 central Caernarvonshire. 



The chief secondary products of the hornblende are magnetite, in 

 dust and in granules, serpentine, often preserving something of the 

 structure of the parent mineral, and sometimes a feebly polarizing 

 pale green substance of the chloritoid family. One slide from 

 Mynydd-y-graig shows, however, the brown compact hornblende 

 passing over into a fibrous mineral presumably actinolite. The 

 hornblende first becomes pale and greenish, and then breaks up into 

 grass-green fibres, which from parallel become divergent, and are 

 seen to be imbedded in a colourless, brightly polarizing mineral 

 which has all the properties of the " secondary enlargement " horn- 

 blende already described. The change is accompanied by a copious 

 separation of granular magnetite, which finally forms a dense 

 border to the altered crystal. The augite of these rocks, being 

 protected by the hornblende, is usually quite fresh. 



All the rocks here described contain augite, as a rule partially 

 amphibolized, and most, if not all, have original hornblende in 

 addition. As some confusion exists with reference to the application 

 of such terms as proterobase and epidiorite, I have judged it 

 advisable to group all these rocks together under the more general 

 title of hornblende-diabase. Their structure is almost without 

 exception that of an ordinary ophitic diabase, but specimens taken 

 at the summit of Mynydd Penarfynydd show a structure which 

 must be called porphyritic in the sense of Eosenbusch*. There are 

 not only two sets of felspars, but the augite also appears of two 

 generations, the earlier one being in idiomorphic crystals. A 

 peculiar aspect is given to some of the more felspathic varieties by a 

 tendency of the larger felspars to collect in patches ; this type of 

 structure is, perhaps, comparable with that which Professor Judd 

 has termed " glomero-porphyritic." 



The hornblende-diabase often contains coarsely crystalline con- 

 temporaneous or segregation-veins. They are on the whole more 

 felspathic than the surrounding rock, and contain crystals of horn- 

 blende up to one inch in length, often very perfectly formed. The 

 geological relations of these rocks will be considered in connexion 

 with those of the hornblende-picrite, which is intimately associated 

 with them. 



VI. The Hornblebtde-Picrite. 



The rock which forms the western slopes of Mynydd Penarfynydd 

 is unique in the district. Professor Bonney has given it the name 

 of hornblende-picrite, and it must be regarded as the type of this 

 well-marked and interesting species. 



The rock occurs in distinct parallel banks to a total thickness of 

 200 or 250 feet, with a dip of 35" or 40° towards S. 30° E., the 

 same as that of the neighbouring Arenig shales. Owing to this 

 inclination, the picrite emerges from the sea under the precipitous 

 cliffs of Trwyn-talfarach, rises on the south-west slope to a little 

 * 'Massigen Gesteine,' new ed., 1886. 



