Charles Schucliert — Historical Geology, 1818-1918. 79 



could not be brought within the limits of the Silurian 

 system, and added, \ ' I believe it to plunge under the true 

 Llandeilo-flags," now placed next below the Bala and 

 above the Arenig, which at the present is regarded as at 

 the base of the Ordovician. 



The Silurian system was defined in print by Murchison 

 in July, 1835, the Upper Silurian embracing the Ludlow 

 and Wenlock, while the Lower Silurian was based on the 

 Caradoc and Llandeilo. Murchison 's monumental work, 

 The Silurian System, of 100 pages and many plates of 

 fossils, appeared in 1838. 



The Cambrian system was described for the first time 

 by Sedgwick in August, 1835, but the completed work — a 

 classic in geology — Synopsis of the Classification of the 

 British Palaeozoic Rocks, along with M 'Coy's Descriptions 

 of British Palaeozoic Fossils, did not appear until 1852- 

 1855. Sedgwick's original Upper Cambrian included the 

 greater part of the chain of the Berwyns, where he said 

 it was connected with the Llandeilo flags of the Silurian. 

 The Middle Cambrian comprised the higher mountains of 

 Caernarvonshire and Merionethshire, and the Lower 

 Cambrian was said to occupy the southwest coast of 

 Caernarvonshire, and to consist of chlorite and mica 

 schists, and some serpentine and granular limestone. In 

 1853 it was seen that the fossiliferous Upper Cambrian 

 included the Arenig, Llandeilo, Bala, Caradoc, Coniston, 

 Hirnant, and Lower Llandovery. On the other hand, it 

 was not until long after Murchison and Sedgwick passed 

 away that the Middle and Lower Cambrian were shown 

 to have fossils, but few of those that characterize what 

 is now called Lower, Middle, and Upper Cambrian time. 



Not until long after the original announcement of the 

 Cambrian system did Sedgwick become aware "of the 

 unfortunate mischief -involving fact" that the most fos- 

 siliferous portion of the Cambrian — the Upper Cambrian 

 — and at that time the only part yielding determinable 

 fossils, when compared with the Lower Silurian was 

 seen to be an equivalent formation but with very dif- 

 ferent lithologic conditions. He began to see in 1842 

 that his Cambrian was in conflict with the Silurian sys- 

 tem, and four years later there were serious divergencies 

 of views between himself and Murchison. The climax 

 of the controversy was attained in 1852, when Sedgwick 

 was extending his Cambrian system upwards to include 



