376 



assume tlie notion that certain beds resembling each other were 



deposited at the same time, and that their extension was 



general. It comports better with our phenomena to contend 



that the Spinatus beds were not merely local deposits, but at 



least so far general in distribution, as to be conterminous with 



the areas of the basins of subsidence ; for during the gradual 



subsidence of a given area their deposits were formed. It is 



true that we are working with an imperfect knowledge so far, 



that these beds have not come down to us in their original 



entirety. From the instability of their petrological character, 



we should only expect to meet with dim traces of the original 



strata, and those only at certain points, where they have escaped 



denuding forces ; from this cause, they have been in some places 



enormously reduced in thickness, and in others, entirely planed 



away. But a consideration of the following particvilars will 



convince us, that with certain exceptions capable of deliberate 



explanation, the persistency of the Spinatus beds is somewhat 



striking. Without generalizing farther on the phenomena, we 



will proceed to adduce facts; mentioning some of the more 



considerable places in each country, where the Zone is exhibited. 



In the Western isles of North Britain, as well as on the 



Eastern coast, we possess most interesting remains of Middle 



Lias, which testify to the scattered archipelagic character of 



the North Jurassic sea. Of the former, the Jurassic rocks of 



Skye and Raasay, we have fortunately all but a complete 



account published of these striking Liassic strata.* Only 



passing notices had been given of such fragmentary deposits 



since 1819, when Dr. Macctjlloch's "Account of the Western 



Islands," first described their true geological structure, until 



our associate Dr. Weight, by bringing his experience and 



knowledge to bear upon the fossils collected by Prof, Geikie, 



(Notes, Quarterly Journal Geol. Soc. 1857, p. 24) from the 



Lias of the Isles of Pabba, Scalpa and Skye, struck the true 



note that led to that harmony and true correlation, for which 



we are indebted to our present clearness of view of the North 



British Lias. Up to this date, 1858, the portions of Lias 



ascertained to exist, embraced the Lower Lias, and the lower 



* See Dr. Bryce's Paper, Quaxterly Journal Geol. Society, London, Aug. 1873. 



