550 Alfred Harher — On some Anglesey Bykes. 



traces often less pronounced. It is later than the original brown 

 hornblende, which it includes. It also includes augite, without any- 

 definite crystallographic relation to it, and so is probably not 

 ' paramorphic' On the whole, it seems likely that this dull-coloured 

 hornblende is a secondary growth posterior in date to the consolida- 

 tion of the rock, though this cannot be regarded with certainty, as 

 in the following case. 



Lastly, we have hornblende, sometimes green, but more commonly 

 colourless, forming a sharply-defined border to original hornblende 

 crystals, and filling the interstices between them. A study of the 

 slides leaves no doubt that this new growth of hornblende-substance 

 has taken place subsequently to the consolidation of the rock. In 

 some cases it can be verified that the new hornblende extends into 

 the space formerly occupied by felspar crystals. Again, the inter- 

 space between two or three original or paramorphic hornblendes is 

 found to be filled by colourless substance, which between crossed 

 Nicols breaks up into distinct portions in crystalline continuity with 

 the several adjacent crystals. This is exactly analogous to the- 

 cement of secondary quartz observed by Dr. Sorby and others 

 between the grains of certain quartzites. Sometimes a well-bounded 

 original crystal has a sharply-defined border of greener horn- 

 blende, and outside this a margin of colourless hornblende- 

 substance with ragged outline abutting upon patches of calcite dust, 

 etc., which probably represent destroyed felspar. In such a case 

 the double border is all in crystalline continuity with the original 

 crystal. The secondary hornblende here described gives rather 

 higher polarisation-tints and a slightly wider extinction angle than 

 the other varieties of the mineral. 



Mr. Van Hise, who was the first to notice the " secondary- 

 enlargement of hornblende fragments " in conglomerates, has re- 

 cently described ^ a like phenomenon in altered diabases from 

 Michigan and Wisconsin, and he cites similar results arrived at by 

 Becke ^ in 1883. I am not aware that this secondary growth of 

 hornblende on the margins of pre-existing crystals has been recog- 

 nized in British rocks, although Prof. Bonney, referring to the rocks 

 now under discussion, has remarked on the sudden transition from 

 dark brown to almost colourless hornblende, and has figured a very 

 characteristic case in a similar rock from Little Knott in Cumberland.^ 

 The Ehodogeidio and Llys Einion dykes furnish the best examples. 



The only remaining mineral that calls for special notice is th& 

 brown mica, which has been mentioned above as biotite, although 

 its characters are not easy to make out very definitely. It is not 

 abundant except in some specimens from Llandyfrydog, which have 

 to the eye almost a lamprophyric aspect. In his specimen [695] 

 especially Henslow remarks the presence of "small shining plates, 

 apparently diallage." These are the flakes of mica, and their 

 peculiar lustre is explained on examination under the microscope.. 



1 Amer. Journ. Sci. May, 1887, 3rd ser. vol. xsxiii. p. 385. 



2 Tscherm. Min. ii. Petr. Mitth. vol. v. part ii. 1883. 



3 Q. G. G. S. vol. xli. plate zvi. fig. 2, 1885. 



