Notifies of Memoirs — W.A.E. JJ&sher — Lr. Devonian Roclis. 511 



Summary. — The main points to be considered in respect of tlie 

 geolo>>ical conditions under which petroleum and gas occur seem to 

 be as follows : — 



1. They occur in rocks of all geological ages, from Silurian 

 upwards. The most productive areas are Palaeozoic in North 



' America, Miocene in the Caucasus. 



2. There is no relation to volcanic action. 



3. The most productive areas for oil in great quantity are where 

 the strata are comparatively undisturbed. Oil, but in less abund- 

 ance, frequently occurs when the strata are highly disturbed and 

 contorted, but gas is rarely so found. 



4. The main requisites for a productive oil- or gas-field are a 

 porous reservoir (sandstone or limestone) and an impervious cover. 



5. Both in comparatively undisturbed and in highly disturbed, 

 areas, an anticlinal structure often favours the accumulation of oil 

 and gas in the domes of the arches. 



6. Brine is an almost universal accompaniment of oil and gas. 



XI. — VULOANIOITY IN LoWER DEVONIAN EoCKS. Thb PrAWLE 



Problem. By W. A. E. Ussher, F.G.S. [Communicated by 

 permission of the Director-General of the Geological Survey.] 



IN the area extending south from the Middle Devonian volcanic 

 series of Ashprington to the Prawle there appears to be no 

 proof of the occurrence of strata older than Lower Devonian. There 

 is no adequate reason for assuming that Lower Devonian rocks as 

 old as the Gedinnian occur on the surface, and there is no certainty 

 that the lowest beds are older than the Lower Coblenzian. 



The occurrence of local volcanic action in Lower Devonian time 

 is proved by a series of diabases and tuffs near Dartmouth, in the 

 Kingswear Promontory, near Stoke Fleming, and in the line of 

 country west from Torcross. 



In association with the northern chloritic band (running from the 

 mouth of the valley on the north of Hull Sands on the east to Hope 

 on the west) we find volcanic materials identical in character with 

 varieties of volcanic rocks associated with the Devonian slates in the 

 line of country west from Torcross ; and here and there in the line 

 of country west from Torcross the volcanic rocks assume a more or 

 less pronounced chloritic aspect. The junction of the slates on the 

 north with the northern chloritic band is a strictly normal one, the 

 chloritic rocks being almost invariably separated from the slates by 

 brown volcanic materials which are everywhere succeeded by the 

 same type of Devonian slate, and in the Southpool Creek and many 

 other sections are found to pass insensibly into the chloritic type. 

 In the Southpool Creek section a hard bluish diabase (? aphanite) 

 occurs in the chloritic band. In the southern chloritic districts of 

 the Prawle the volcanic rocks may still be here and there detected 

 by texture or colour. Volcanic rocks occur in the mica schists of 

 the Start coast, and can be detected even when only a few inches in 

 thickness. At Spirit-of-the-Ocean Cove chloritic rock with much 

 calcspar occurs in association with tuffs and a grey rock with 



