Silurian Bocks. 81 



of formations, and shall begin with the loWest fossiliferous 

 deposits. 



Silurian Mocks. 



It is certainly remarkable, considering the short time that 

 has elapsed since Sir R. Murchison first proposed the sepa- 

 ration of the lower beds of tlie palaeozoic strata into one great 

 series, that rocks which appear to be clearly made out to be- 

 long to the Silurian system should have been already recog- 

 nised in so many regions remotely distant from each other. 

 That they constitute a great part of Europe has been shewn 

 by many writers/ The geologists of the United States and 

 Mr Lyell have told us how widely they are spread over the 

 northern States of North America ; and we learn from Cap- 

 tain Bayfield that they occur extensively all round Lake 

 Huron ; northward towards Hudson's Bay ; along the north- 

 ern side of the valley of the St Laurence, eastward to the 

 Strait of Belle Isle, and on the western coast of Newfound- 

 land from that strait to its southern extremity. M. Alcide 

 D'Orbigny has described them as extensively developed in 

 South America ; and from Mr Darwin we learn that they 

 probably exist in the Falkland Islands, adjoining the farthest 

 extremity of that continent. It is also more than probable, 

 from the information we already possess, that they exist in 

 Australia. The rocks were known, and had been partially 

 described, but they were not understood ; they were known 

 minerailogically, and deposits separated by great intervals 

 of time were classified together, under the vague, uncertain, 

 general term of graywacke, orgraywacke-slate, or clay-slate, 

 The clear development of the system, and lucid descriptions 

 of the normal types in the Silurian region of Britain, dispelled 

 the obscurity that hung over the history of these ancient 

 beds ; and now geologists are at work in all countries, mak- 

 ing out the great features of resemblance, and registering 

 those variations in mineral and fossil contents, dependent on 

 geographical position and other loca.1 causes, which are found 

 to prevail more or less in all formations. 



It appears to be now the opinion of those geologists who 

 have most carefully and ex.tensively studied th^ sedimentar 



VOL. XLI. NO. LXXXI. — JULY 1846. F 



