A. Somerrail — Banded Hocks of the Lizard District. 505 



The Keuper formation consists of beds of sandstone and marl, 

 and the lower part is sometimes so like the Pebble-beds of the 

 Bunter that it is difficult to distinguish one from the other in the 

 absence of an extended section, showing the relative position of 

 the formations. 



The Keuper has fewer pebbles, but thicker beds of shale and 

 marl. The general opinion is that it was deposited in a series of 

 lakes, for no fossils indicating marine conditions have been found, 

 and locally only a few land-plants at Flay brick and Storeton. 

 There were shores or banks of sand and clay, and the occurrence 

 of ripple-marks, rain-marks, sun-cracks, worm-tracks, and foot- 

 prints indicate changes that might be expected in such situations. 



In the Red Marl there is abundant evidence of a salt-lake or 

 lagoon, and the continuous evaporation is obvious from the thousands 

 of pseudomorphic crystals of chloride of sodium that cover the 

 surfaces of shaly beds in the lower portion, while in Central 

 Cheshire thick beds of rock-salt afford evidence of the entrance of 

 sea-water and the subsequent precipitation of the salt. These 

 conditions must have continued for a long period, and it is quite 

 possible that the surrounding country may have been of the desert 

 character that some geologists suppose, though there seems very 

 little to support such a conclusion. 



The whole of the Triassic strata were deposited in a subsiding 

 ai'ea, but there is no reason to suppose that the water was deep, tor 

 the frequent presence of current-bedding indicates shallow-water 

 conditions. It may be remarkahle that subsidence should accompany 

 deposition, but if there were no subsidence it is difficult to conceive 

 how deposits of great thickness could be formed in lakes or near 

 the land, for the sediment would be spread further out over the 

 shallow sea-bottom. Dr. C. Ricketts, F.G.S., is of opinion that the 

 increasing weight of the sediment is the cause of the subsidence, 

 but it is just as easy to suppose that where a great thickness of strata 

 occurs, it is in consequence of a long-continued subsidence from 

 some other cause which rendered the great accumulation possible. 



IV. — On the Nature and Origin of the Banded Structure in the 



Schists and other Rocks of the Lizard District. 



By Alexander Somekvail, Esq. 



Introduction. 



TN my last communication, " On the Schists of the Lizard District," 

 published in this Magazine for April, which was meant as 

 preparatory for the present paper, my object was to show that the rocks 

 known as the " talco-micaceous," " hornblendic," and "granulitic" 

 groups were of true igneous origin, also plutonic ? and had been 

 formed out of a common, but complex magma, and that all of 

 these groups were made up of rocks differing widely from the types 

 of each of them, which in some instances were seemingly due to 

 differences in the rate of cooling, and to chemical affinity ; and yet 

 again in other instances to subsequent mechanical movements and 

 pressure during or after consolidation. 



