506 ME. B. HOBSON ON THE BASALTS AND 



The presence of quartz (or ' glassy feldspar ') in the rocks of 

 Stone, Knowle, and Western Town is mentioned by Mr. Etheridge 

 in Mr. Yicary's paper before cited. It was previously noticed by 

 De la Beche as follows^ : — " Above Ideston and Knole we find por- 

 phyries sometimes containing crystals of quartz, at others of felspar, 

 and occasionally of both together — a very interesting fact, inasmuch 

 as it is the only locality with which we are acquainted where any 

 portion of those quartziferous porphyries, of which fragments are 

 so commonly found in the red conglomerate between Exeter and 

 Teignmouth, occurs in place;" and again (p. 217) *' the quartziferous 

 porphyry near Dunchideock." I hold that De la Beche was misled by 

 the quartz inclusions in these basalts into regarding them as quartz- 

 porphyries. 



A mass of genuine quartz-porphyry measuring about 4 feet x 1 foot 

 is exposed on the west side of the road opposite to the entrance-gate 

 of Dunchideock House, but it appears to belong to the New Eed 

 Breccia. 



Dr. Hatch has described ^ some of the rocks which I term olivine- 

 basalts as porphyrites (andesites), viz., Western Town, Ide (his 

 No. 943 = my No. 913), Quarry N.E. of Knowle, N.E. of Holcombe- 

 Burnell (his No. 946), and Knowle Quarry, W. of Dunchideock 

 (his No. 949). It is difficult to identify the localities of Nos. 946 

 and 949. If the Geological Survey and 6-inch Ordnance maps are 

 to be trusted, there is no ' trap ' N.E. of Holcombe-Burnell. Probably 

 S.E. of Holcombe-Burnell is meant, in which case Dr. Hatch's 

 No. 946 is probably the same as my No. 912. No 'Knowle 

 Quarry' is shown on the maps W. of Dunchideock. Probably 

 that at Orchard Copse (my No. 916) is meant. 



Obvious pseudomorphs after olivine phenocrysts are plentiful in 

 No. 916, but in No. 913 only one red pseudomorph 0*47 millim. in 

 diameter and in No. 912 b only two above 0*3 millim. in diameter 

 are present, though smaller ones are numerous in No. 912 b ; but in 

 Nos. 912 A, B, c, and 913 magnetite pseudomorphs about 0-09 millim. 

 in diameter, which I take to be after olivine (as previously described), 

 are abundant. 



The rock of Long Lane, Loxbear (No. 891), differs so much from 

 the rest as to deserve separate description. The hand-specimen is 

 whitish and very micaceous. Microscopically the rock is seen 

 to contain pseudomorphs after idiomorphic olivine, consisting of 

 brownish alteration-products outlined by black opaque iron ore ; 

 brown mica (very abundant), apatite in short prisms, much opaque 

 iron ore, and felspar. The mica is often allotriomorphic towards 

 the felspar, but sometimes the reverse relation appears to obtain. 

 It is very full of inclusions of haematite, black as seen under a low, 

 but red translucent under a high power, often arranged in a series 

 of lines crossing one another at about 60° as seen in a basal pina- 

 coid section. These are probably pseudomorphs after magnetite, as 



1 Eeport, pp. 203, 204. 



2 Notes to Mr. Ussher's paper, Geol. Mag. for 1892, p. 250. 



