Ill] ORDOVICIAN ROCKS. 37 



oldest fauna, of which the ancestral types have yet to be 

 discovered. During the last few decades the number of 

 Cambrian fossils has been considerably increased, and in 

 certain regions of North America and China there are found 

 many thousand feet of strata above the typical Archaean rocks 

 and below the newer fossiliferous beds of Cambrian age. It 

 is reasonable to suppose that future research may extend the 

 present limits of fossil-bearing rocks below the horizon, which 

 is marked by the occurrence of the widely distributed and 

 oldest known trilobite, the genus Olenellus. 



The vast thickness of Cambrian strata was for the most 

 part laid down on the floor of a comparatively deep sea ; other 

 members of the series represent the shingle beaches and coast 

 deposits accumulated on the slopes of Archaean islands. 

 There have been many conjectures as to the distribution of 

 land and sea during the deposition of these rocks; but the 

 data are too imperfect to enable us to restore with any degree 

 of confidence the physical geography of this Palaeozoic epoch, 

 of which the sediments stood out as islands of Cambrian land 

 during many succeeding ages. 



III. Ordovician. 



Since the days when Sedgwick and Murchison first worked 

 out the succession of Palaeozoic strata in North Wales, there 

 has always existed a considerable difference of opinion as to 

 the best method of subdividing the Cambrian-Silurian strata. 

 Later research has shown that the rocks included by Sedgwick 

 in his Cambrian system, fall naturally into two groups; for 

 the upper of these Prof. Lapworth has suggested the term 

 Ordovician, from the name of the Ordovices, who inhabited a 

 part of northern Wales. At the base of the system we have a 

 series of volcanic and sedimentary rocks to which Sedgwick gave 

 the name Arenig ; above these there occur the Llandeilo Flags, 

 succeeded by a considerable thickness of rocks known as the 

 Bala series. The rocks making up these Ordovician sediments 

 consist for the most part of slates, sandstones and limestones with 



