Dr. 0. A. L. Morch — On the Crag of Iceland. 391 



formed in strata incapable of being stretched, which occur among 

 strata capable of being stretched by the weight of a mass of rock 

 displaced by a fault. Fig. 2 is intended to show two wants, amount- 

 ing together to about 73 yards, caused by the fault above mentioned, 

 which had a maximum downthrow of 220 yards ; its zero points 

 being one mile distant. 



It is by no means asserted that even in the case of shales the capa- 

 bility of being stretched is without limit. In every case after the 

 limit is reached, the fault can only increase its downthrow by in- 

 creasing in horizontal length {i.e. adding to the distance between its 

 two zero points), or by producing on its downthrow side one or more 

 faults along lines of weakness forming angles with its own line. 

 Such lines of weakness, destined to become faults, will be found in 

 the largest wants in the thickest strata. Faults which disturb 

 stratified deposits of varying composition are very frequently filled 

 with a "hash" of the softer strata only, such as shales and fire- 

 clays ; and this circumstance may be due in some cases to the fact 

 that the fault has been made (during the increase of a master-fault) 

 along a line from which the harder beds — of sandstone or the like — 

 had been previously withdrawn. 



When a master-fault, after exhausting the capability of being 

 stretched possessed by the strata displaced, produces smaller faults, 

 the latter will have their downthrow on the side towards which 

 the master-fault increases. The superincumbent strata will settle 

 down along the lines of the master and smaller faults till the gap 

 which would otherwise have existed is filled up. 



III. — On the Mollusca of the Crag-foemation of Iceland. 



By Dr. 0. A. L. Morch, For. M. E. Soc, Edin. ; For. Corr. Z. S., Loud. ; 

 Acad. Sc, Philad. 



THE fossils of the volcanic Islands of the Atlantic Ocean, St. Helena, 

 Madeira, Teneriffe, and Iceland, have a special interest, as giving 

 dates relative to the supposed Atlantic Continent, The fossil 

 land-shells of St. Helena and Madeira appear to belong to a period 

 corresponding to the Crag-formation of England. The marine 

 fossils of the Canaries, on the contrary, belong to a somewhat more 

 ancient period. 



In Iceland beds of fossils are found in several places, but belong- 

 ing to very difi"erent geological ages. The most celebrated forma- 

 tion is the Surturbrand, containing a fossil flora, chiefly known from 

 the description of Prof. 0. Heer of Zurich, from the collections of 

 Prof Steenstrup, etc. It contains about thirty-seven species of 

 plants, chiefly belonging to Araucarites (A. Sternhergi, with stems 

 from 1^ to 2^ feet in thickness) , Betula, Corylus, Alnus, Vitis, Lyrio- 

 dendron, TJlmus, Bhamnus, Rhus, Quercus, etc. A wing of a Carabus 

 is the only trace of animals. The author considers this formation to 

 be of Miocene age, but belonging to two different periods. 



The fossil shells beloag to a more recent period; but as they are 

 never found in the same place in superposed strata, it is diflicult to 



