284 



Leicestershire. Messrs. HILL & BOMS' EY (1) have shown that the pre- 

 Triassic stratified rocks of Cham wood Forest are very largely com- 

 posed of volcanic material and that many of them are true volcanic 

 breccias. Lapilli of andesitic lava and broken felspar-crystals are 

 frequently recognizable in the thin sections. The ferro-magnesian minerals 

 have been generally decomposed, but in the rock occurring near the cottages 

 east of the monastery, in the northern portion of the district, Messrs. 

 HILL and BONNEY observed lapilli containing plagioclase and the black- 

 bordered hornblende so characteristic of the hornblende-andesites. 



North Wales. Volcanic rocks of intermediate composition will prob- 

 ably be found to occur on a much more extensive scale than is at present re- 

 cognized. Mr. TAWNEY< 2 > has shown that the hill of Carn Boduan in the Lleyn 

 peninsula of Caernarvonshire is mainly composed of hornblende-porphyrite. 

 The least altered rock is almost black in colour. The porphyritic constituents 

 are plagioclase, green hornblende and a little augite. The ground-mass 

 is formed of felspar-microlites, much dusty magnetite and a little inters- 

 titial glass. Apatite is abundant. Crystals of iron oxide are often 

 associated with the hornblende. The freshest varieties of this rock 

 almost deserve the name andesite. The mass appears to be intrusive in 

 Ordovician strata. 



Shropshire and Montgomeryshire. Mr. WATTS (S) has described a 

 series of andesitic lavas occurring in the Breidden Hills. They are 

 interstratified with Ordovician sediments. The rocks are of a dark 

 grey or dull greenish colour. The average specific gravity is about 

 2'66. The porphyritic constituents are felspar and pyroxene, the 

 crystals of the former rarely exceeding lin. in length, those of the 

 latter being smaller. The ground-mass is a "mikrolithen filz." Owing 

 to alteration it is difficult to be certain as to the presence of inters- 

 titial glass. The porphyritic felspars are often turbid in thin sections. 

 The pyroxene is mainly rhombic ; but monoclinic augite, often more or 

 less altered to chlorite, is also present. The enstatite is green and evi- 

 dently therefore somewhat altered. It resembles the enstatite (bastite ? ) 

 of the pyroxene-porphyrite of Eycott Hill. Iron-ores are present. 

 Brown hornblende was recognized in one or two slides. 



South Wales. Altered andesites occur at several points in South Wales 

 but have not as yet received much attention. A pale- coloured andesitic 

 or trachytic breccia is exposed near Trefgarn Bridge, Pembrokeshire, 

 beneath shales which contain Lingula Flag fossils. A somewhat basic 

 porphyrite occurs at Crab Hole near Dale, also in Pembrokeshire. Por- 

 phyrites also occur on Skomer Island but they belong to the border line 

 between basic and intermediate rocks. 



Scotland. Rocks of the group now under consideration are very 

 extensively developed in certain portions of Scotland. They constitute 



(1) Q.J.G.S., Vol. XXXIV. (1878), p. 19J 



(2) G.M., 1882, p. 549. 



(5) Q.J.G.S., Vol. XLI. (1885), p. 532. 



