3T4 TRIASSIC VOLCANIC ROCKS OF DEVON. 



Killerton Park has the aspect of a great volcanic centre formed of 

 compact dark micaceous lava, vesicular in the higher portions, becom- 

 ing red, scoriaceous, and ashy in the neighbouring quarries at Bud- 

 lake and Silverton. The compact rock near Tiverton at Holmead, 

 almost like a mica schist for its abundance of mica, similarly corre- 

 sponds to the scoriaceous and ashy beds which extend from Washfield 

 to Loxbeare. The compact lava which forms Knowle Hill may repre- 

 sent the centre from which the porous lavas and ashes of Pocombe and 

 Northernhay flowed. In the Crediton district, Postbury furnishes the 

 compact crystalline rock which thins as it extends to Yeotown, while 

 highly vesicular rocks are met with at and about Spence Combe, often 

 jointed with "mountain cork." As in all similar rocks, the vesicles, 

 now amygdaloidal, are greatly elongated by flowing movement of the 

 rock, so as to often give an aspect of stratification. Other large vol- 

 canic masses occur near Haldan and at North Tawton. Their colour 

 is dark red or purple black. They vary in mineral composition ; are 

 remarkable for a clear felspar, which occurs in porphyritic masses 

 in the rock at Knowle and many other places ; and this is associated 

 with plagioclase, with a frequent abundance of brown mica, some 

 orthoclase, potash mica, some brown hornblende, and much hematite. 

 Hence some of these rocks seem to be related to the minettes. Some 

 Exeter rocks are referred to by Professor Bonney as basalts. 



Tertiary Volcanic, Roclcs. 



The British volcanic rocks of Tertiary age cover two distinct 

 areas ; first, the large district in the N.E. of Ireland extending 

 round Lough Neagh, which comprises nearly the whole of Antrim 

 and the adjacent part of Londonderry; and, secondly, the chain 

 of the Inner Hebrides, including Mull, Rum, Eigg, Canna, Muck, 

 and three-fourths of Skye. This line of old volcanic activity 

 extends N. to the Shiant Islands, and appears again in the Earoe 

 Isles, before terminating in the older volcanic districts of Ice- 

 land. Hence, the tertiary volcanoes of Britain are the southern 

 end of a band nearly 800 miles long, which is still active at its 

 northern extremity. The rocks in Ireland and in Scotland include 

 acidic series as well as basic series. The basic series of Scotland 

 is demonstrably the younger ; but in Ireland the rocks which have 

 been identified as trachyte and rhyolite appear to be older than the 

 basalts. 



Acidic Rocks of the North of Ireland. The acidic rocks of 

 the North of Ireland have hardly received the attention which their 

 importance demands. They occur in many places, especially near 

 Tardree, in the neighbourhood of Templepatrick, near Broughshane ; 

 and at Ballyknock, south-west of Hillsborough. Some of these 

 rhyolites are regularly bedded, and intrusive in the dolerites, and 

 therefore younger than the dolerites. The analysis by Mr. Hard- 

 man does not differ from the composition of typical rhyolites of 

 Germany. 





