330 SECTIONAL TRANSACTIONS.— C. 



Afternoon. 

 Excursion to Avon Gorge. 



Wednesday, September 10. 



Dr. G. W. Tyrrell (Glasgow) and Dr. K. S. Sandford (Oxford). — 

 Tectonic Relations and Petrography of Spitsbergen Dolerites. 



In early Cretaceous times the eastern part of the Spitsbergen region was the theatre 

 of great igneous activity which took the form of dolerite intrusions, both as dikes and 

 more massive vertical masses, and as sills of considerable extension, thickness and 

 uniformity, injected into horizontal sediments of Late Palseozoic and Mesozoic ages. 

 Vertical intrusions occur on the grand scale in the central and southern parts of the 

 Hinlopen Strait, and in the northern part of the Stor Fjord region. Sills are also very 

 frequent in these localities, and extend far south in the Stor Fjord, west in the Sassen 

 Bay region, and east into North-east Land. 



A close relationship subsists between the fjords and the dolerites. The lines of 

 weakness which delimit the Kainozoic fjord-blocks were already in existence as 

 fissures and fractures in Lower Cretaceous times, and provided channels for the 

 ascent of dolerite magma. But not till long after the dolerite had solidified did the 

 fracture-lines develop as fjord-faults. The Kainozoic crustal instability in this region 

 was probably caused by the enormous displacement of doleritic magma at an earlier 

 period, the latter itself probably being due to the overloading of the East Spitsbergen 

 continental shelf by sedimentary accumulations. The basalt floods of the North 

 Atlantic or Thulean region in early Kainozoic times may represent a later expression 

 of the same crustal instability that produced the Spitsbergen dolerites and the corre- 

 lated fjord-faulting. 



Petrographically the dolerites are of a common type, ranging from gabbroidal 

 varieties in the larger intrusions to basaltic varieties in the smaller dikes and on the 

 edges of larger masses. The majority of the rocks are of medium grain, and may 

 contain olivine, or quartz, or both, in addition to labradorite, enstatite augite, and 

 titaniferous iron ore. An unusually interesting petrographical feature is the occurrence 

 of olivine side by side with quartz in the form of a micropegmatitic mesostasis. The 

 best example is a gabbroidal type from the Alk Range in Hinlopen Strait, in which 

 fresh ohvine partially or wholly enclosed in plates of augite co-exists alongside an 

 abundant mesostasis of micropegmatite. Somewhat similar rocks occur in the sills of 

 the Stor Fjord region. These rocks therefore provide good examples of Bowen's 

 reaction relation operating in a doleritic magma. 



Dr. H. C. Versey. — The Speeton (pre-glacial) Shell Bed. 



Mineralogical analysis reveals a large percentage of minerals which may be attri- 

 buted to Scandinavia and the rock is considered equivalent in age to the transported 

 masses of sand in the Yorkshire Drift, i.e. Upper Pliocene. It possibly represents a 

 period of relatively genial climate preceding the Basement glaciation and succeeding 

 an earlier glaciation which did not reach the British coast. The bed is regarded as 

 in situ, owing its present elevation to subsequent uplift. The local geography at the 

 time is also discussed. 



Dr. A. Heard and Mr. J. F. Jones. — Eohepatica dyfriensis, a Livenvort- 

 like Plant from the Loiver Downtonian of the Llandovery District. 



This peculiar organism occurs as a thin dorsiventral thallus with an apparently 

 gregarious habit. No complete specimens have been obtained, but the largest frag- 

 ment collected exceeds 10 x 8 cms. 



Chemical and mechanical methods of treatment have shown the thallus to be 

 highly differentiated. 



The plant bears many ovoid bodies about -4 cm. in diameter, from which numerous 

 disc-like spores (?) have been recovered. 



The dorsal surface bears groups of numerous air-pores, the cells of which contain 

 plastids ; and are separated from each other by only one or two epidermal cells. 



