22 LIASSIC SYSTEM. 



III.— LIASSIC SYSTEM. 



The shallow water in which the Ehsetie sediments were 

 deposited became gradually converted into a deeper sea, on the 

 floor of which were accumulated the regular Liassic strata of 

 limestones and shales. Liassic rocks stretch across England from 

 the mouth of the Tees to the coast of Dorset, and occur as 

 isolated patches in Shropshire and Cumberland; exposures of 

 Liassic beds are met with also in Sutherlandshire, in the west 

 of Scotland, and on the Antrim coast. The strata belonging to 

 this system which form the bold headland of HuntcHff, near 

 Saltburn, and the cliffs of Eunswiok Bay and of other parts of 

 the Yorkshire coast, are essentially marine in origin. Blocks of 

 wood converted into jet in the 'jet-rock' of the Upper Lias, 

 and fragments of silicifled wood from a lower horizon, serve as 

 records of the vegetation that flourished on the land bordering 

 the Liassic sea. The Lower Lias strata of Dorsetshire exposed 

 in the cliffs near Lyme Eegis are rich in remains of marine life, 

 and from them have also been obtained the majority of British 

 Liassic plants. It is a common occurrence to find an abundance 

 of land-plants entombed in marine sediments, and even in strata 

 that have been deposited in comparatively deep water we meet 

 with traces of terrestrial forms. In a short account of dredging 

 operations in the Eastern Pacific contributed to Nature in 1892,' 

 Alexander Agassiz speaks of an immense amount of vegetable 

 matter having been dredged from depths of 1,500 fathoms ; there 

 was hardly a haul -without much water-logged wood and more or 

 less fresh twigs, seeds, fruits, etc., in all stages of decomposition. 

 It is under similar conditions to these that most of the British 

 Liassic plants have been preserved. The Lias strata are usually 



' Agassiz (92). 



