RECENT STUDIES IN THE GEOLOGY OF THE FAEROES 37 



and sills, occur, as indicated by Fig. 1, in two different intrusion levels: a 

 lower one, where the Coal-bearing series and the Tuff-Agglomerate zone have 

 acted as a zone of weakness ; and an upper one, where the boundary between 

 the lower and the upper zones in the Upper Basalt sequence apparently 

 acted as a zone of weakness. 



Since the dykes have been observed as irregular intrusive formations as 

 well as sills, we must assume that all the above forms of intrusions belong 

 roughly to the same phase of eruption. 



Irregular intrusive formations. The irregularintrusive formations are intruded 

 in the Coal-bearing series and especially in the overlying Tuff-Agglomerate 

 zone. Since the latter is often strongly porous and not very resistant to 

 magma pressures in different directions, these intrusions will have quite an 

 irregular shape. Such irregular intrusive formations appear in the north- 

 easterly part of Suduroy, onTindholmur, and in the western most part of Vagar. 



On Suduroy they are present at the coast on the northern side of Hvalbiar- 

 fjordur and along the eastern coast of Suduroy to Hvannhagi, as shown by 

 Fig. 2. On the northern side of Trongisvagur they can be studied in ravines in 

 the vegetation-covered terrain. They are also visible on the south side of the 

 fjord at the easterly boundary of the Coal-bearing series. In the north the 

 intrusive formations reach a height of up to 35 m above sea level, and in the 

 south they attain thicknesses of about 100 m at some places. By the intrusion 

 the easterly coal-area has been strongly dislocated and destroyed with 

 regard to coal mining. Occasionally long apophyses are visible in the Upper 

 Basalt sequence. 



On Tindholmur and on the westernmost part of Vagar the irregular 

 intrusive formations — like on Suduroy — are intruded in the Coal-bearing 

 series and in the overlying Tuff-Agglomerate zone. 



Sills. Sills occur on Streymoy, Eysturoy, Svinoy, and Fugloy (Figs. 4-6). 



The large Streymore sill (Fig. 4) extends in an almost true NW.-SE. 

 direction. It is about 9 km long and varies in width from 1 to 2 km in the 

 north and from 2 to 3 km in the south. At one place the erosion has been so 

 deep that the sill has been divided into a northerly and a southerly part. The 

 lowest position of the sill is along its central part in the west, about 300 m 

 above sea level, where it more or less follows the regular basalt bedding. To 

 the north, south and east it is strongly transgressive. The thickness is greatest 

 towards the west, up to about 60 m, whereas it is strongly decreasing towards 

 the east, down to 10 m, in some parts even to 5 m. 



The Eysturoy sill (Fig. 5) extends about 6 km in a NW.-SE. direction and 

 ranges in width between 2 and 3.5 km. It is most conspicuous towards the 

 west where it forms prominent columnar structures. Like the Streymoy sill, the 

 lower sill boundary on the west side has a smooth, winding course, whereas in 

 the north, south, and east it is strongly transgressive. Again, like the Streymoy 

 sill, the Eysturoy one has its lowest position and greatest thickness — up to 



D 



