possessing the reticulated structure so common in the lower Paleozoic 

 diabases, appears on the other hand to be somewhat more abundant. 

 Chemical analysis shows that the iron is about equal in the rock and in the 

 veins, but that the lime and magnesia are more abundant in the former 

 than in the latter. The iron of the veins is mainly present in ores, not in 

 ferro-magnesian silicates. 



The veins exhibit the most perfect illustrations of micro-pegmatite that 

 can possibly be imagined. The rock of which they are composed is in fact 

 a granophyre. The felspar intergrown with the quartz is generally turbid so 

 that no difficulty is experienced in separating the two minerals. The ferro- 

 magnesian constituent of the veins is augite. Enstatite is rare if not entirely 

 absent. Now on comparing the veins with the rock it will be noted at once 

 that they consist of the minerals which formed last ; in other words they con- 

 sist of augite, orthoclase and quartz. Iron-ores separate at different times 

 in the process of consolidation and may therefore be left out of account. 

 These facts point strongly to the conclusion advocated by Mr. WALLER, 

 that the veins are of the kind known as contemporaneous. They are 

 composed of the mother liquor left after the separation of the more basic 

 compounds. The occurrence of augite is conclusive as showing that the 

 veins are igneous products and not exfiltration veins formed after the 

 complete consolidation of the mass. In mineralogical composition these 

 veins are related to the augite-granophyres of Carrock Fell, Barneveve 

 Mountain (Carlingford district in Ireland) and Mull. 



At the margins of the intrusive mass the rock of Penmaenmawr tends 

 to become compact and porphyritic. Mr. WALLER states that a slice of the 

 compact variety is almost entirely made up of interlacing felspar crystals 

 with occasional porphyritic ones ot larger size. He thinks that in the 

 interstices there may be a small amount of residual glass. It is thus 

 evident that the rock at its margins assumes a trachytic texture. The 

 normal holo-crystalline rock is the plutonic equivalent of the enstatite- 

 augite-andesites hereafter to be described. The following analyses by 

 Mr. PHILLIPS represent the composition of different varieties of the normal 

 rock. The analysis by Mr. WALLER represents the composition of one of 

 the grey veins. An analysis of one of the Klausen rocks is inserted by 

 way of comparison : 



I. II. III. IV. V. 



Si(X ... 58-45 ... 62-24 ... 61 "75 ... 65-1 ... 59'97 



Al.,0, ... 17-08 ... 18-25 ... 18-88 



Fe.,0 3 ... -76 ... 1-05 ... -52 



FeO ... 4-61 ... 3-08 ... 3-52 



MnO ... tr. ... tr. ... tr. 



CaO ... 7-60 ... 4-00 ... 3-54 



MgO ... 5-15 ... 2-27 ... 1-90 



K.,0 ... 1-02 ... 1-49 ... 1-24 



Na.,0 . 4-25 3-79 3-67 



12-9 ... 16-93 



2-0 ... 2-41 



4-7 ... 4-83 



tr. 



4-7 ... 5-10 



2-8 ... 3-61 



3-9 ... 1-32 



2-8 3-87 



PA tr. ... tr. ... ti-. 



FeS., ... ... -39 .. -09 



H 2 0" . . 1-07 ... 2-69 ... 4-46 ... 1-9 ... _1'60 



99-99 99 T 44 99 -57 10O8 1)9-64 



Sp. Gr. 2-94 2'75 2'79 2'72 



