Intelligence and Miscellaneous Articles. 521 



(i.) Granite occupying the northern and north-western part of 

 the district. The ordinary type is a fairly normal biotite-granite ; 

 but a variety at Meillionydd shows an exceptional structure, the 

 biotite moulding the other constituents in ophitic fashion. The 

 granites are intrusive in the Arenig shales. 



(ii.) Gabbro, diorite, &c. in two small patches only. The rock, 

 originally a gabbro, passes into diorite, the diallage becoming am- 

 phibolized, and the iron-ores disappearing with the production of 

 granular sphene. Near a bounding-fault this hornblendic rock 

 becomes locally schistose and gneissie. 



(iii.) Diabase, in the centre, forming the mass of Mynydd-y- 

 Ehiw, and occurring in dykes and sheets near Sarn. 



(iv.) Hornblende-diabase showing various relations between the 

 augite and hornblende. Besides the conversion of the former 

 mineral to the latter, a closely similar hornblende has grown as an 

 original border to augite-nuclei. The " secondary enlargement " 

 of hornblende-crystals is also exhibited. 



(v.) Hornblende-picrite in several varieties, forming stratiform 

 banks to a thickness of 250 feet and surmounted by hornblende- 

 diabase. The two rocks seem to be in close relation to one another, 

 and to have been injected as laccolites between the Upper Arenig 

 strata near Penarfynydd and Ehiw. 



(vi.) Dolerite-dykes cutting all the other rocks, and probably 

 Post-Carboniferous and Pre-Permian. 



With the exception of the last all these rocks were referred, on such 

 evidence as is available, to the Bala age. 



LXVI. Intelligence and Miscellaneous Articles. 



MOUNTAIN FOKMATION. 

 To the Editors of the Philosophical Magazine and Journal. 



G-ENTI/EMEN, 



T AM glad that Prof. Le Conte has called attention to the views of 

 ■*- American geologists regarding the boundaries of the Appalachian 

 Chain. When I suggested* that the Appalachians were only 

 apparently an exception to the general rule, that a great mountain- 

 range consists of a central gneissie core with largely involved rocks 

 on either flank, I was aware that many American geologists con- 

 sider that the folded sedimentaries making up what Prof. Le Conte 

 calls the " Appalachians proper," were originally laid down against 

 continental land, of which the crystalline rocks to the eastward are 

 the remnants. 



Though much indebted to the ideas of American geologists, I 

 confess that this is one I have never been able to assimilate. 



In what respects do these crystalline rocks differ from those of 

 the Alps and other mountain-ranges in which they form the core ? 

 They are both folded with the sedimentaries, and although deuu- 



* " The Geological Consequences of the Discovery of a Level-of-no- 

 Strain in a Cooling Globe" (Phil. Mag. March). 



