RECENT''sTUD1ES in THE^GEOLOGY of the FAEROES 35 



At the transition between the two areas several coal bands often occur. The 

 total thickness of the two coal bands ranges generally from 50 to 150 cm with 

 an average thickness of f m for the western part of the area. Towards the 

 east and north the coals taper out. 



The coals must be classified somewhere between lignite and bituminous, 

 and appear as two types: glossy (Vitrite) and dull (Durite). The glossy coals 

 are lustrous, hard, with conchoidal fracture and very pure. Their caloric 

 value lies between 6000 and 6300 kcal; the ash content being below 5 per cent. 

 The dull coals are streaky, brittle, crumble easily, and are less pure. Their 

 caloric value lies generally between 5000 and 5500 kcal and the ash content is 

 often close to 20 per cent, sometimes even higher. 



Leaf fossils are very scarcely represented, possibly because the coals are 

 allochtonous. As mentioned above, only two definable species have been 

 found: Sequoia langsdorfii and Taxodium distichum. 



THE TUFF-AGGLOMERATE ZONE 



After the long rest period renewed volcanism set in, and the eruptive 

 activity in the intial stage was highly explosive with a production of predomin- 

 antly pyroclastic material. The deposits of pyroclastics overlying the Coal- 

 bearing series and underlying the Upper Basalt sequence have been described 

 as the Tuff-Agglomerate zone (Fig. 1). 



The Tuff-Agglomerate zone occurs on the east side of Suduroy, on Tind- 

 holmur, and on the west side of Vagar. On Suduroy (Fig. 2) it appears as an 

 elongated beh, 2 to 3 km in width and about 10 km long, from the north side of 

 Trongisvagur. It can be studied in ravines and in coastal cross-sections, where 

 it is evident that it overlies the Coal-bearing series and is overlain, some- 

 times with interbeddings, by the Upper Basalt sequence. Since the deposit 

 almost entirely consists of pyroclastic materials such as ash, lapilli, and volcanic 

 bombs, the thickness varies considerably from place to place. Generally 

 it is 20 to 30 m. However, thinner as well as much thicker layers occur. 



It is hkely that the Tuff Agglomerate zone on Suduroy and on Vagar, 

 where the conditions are quite analogous, covers the eruption fissures which 

 have been feeding the Lower Basalt sequence, as well as their closest surround- 

 ings. Furthermore, since they appear along and follow the direction of the 

 Suduroyarfjordur and the Mykines fjordur, it is tempting to suppose — as will 

 be discussed later on — that the orientation of the inter-island straits is 

 determined by the longitudinal bearing of these eruption fissures. 



THE UPPER BASALT SEQUENCE 



The explosive eruption activity was succeeded immediately by a volcanism 

 which resulted in the formation of the Upper Basalt sequence (of. Fig. 1). This 

 is most likely what happened, since the Tuff-Agglomerate zone, at the bound- 

 ary of the Upper Basalt sequence, alternates with thin beds belonging to this 



