170 LOWER SILURIAN OF CAPE BRETON — GILPIN. 



points ill the districts under consideration, that the original 

 thickness varied with the conditions of deposition, which would 

 be paralleled by the facts observable among the overlying Basal 

 Carboniferous rocks. 



The exact position of these measures in the Geological Scale is 

 not yet determinable with absolute certainty. When compari- 

 sons are made between geological horizons in Nova Scotia and 

 those further west, or on the western side of the Continent of 

 Europe, it is found that the general conditions characterizing 

 such horizons on one side or the other do not necessarily prevail 

 in Nova Scotia. Local peculiarities of surrounding land, and 

 duration and conditions of deposition, have produced such changes 

 that the geologist can but say, so far as can be judged, such and 

 such a series corresponds best with such and such a group. 



Dana, in his Geology, gives an excellent account of the Pots- 

 dam period, then regarded as the base of the Lower Silurian, and 

 the geological secjuent to the Azoic period, the period preceding 

 the appearance of animal life. Since then there has been 

 introduced horizon after horizon, until, Ijetween the base of 

 his Lower Silurian and the true Azoic, there stretches now a 

 long list of measures. Thus Sir J. William Dawson, writing 

 about a year ago, places in descending order, below the Silurian, 

 the Ordovician, embracing the Cobequid Series, &c., and the 

 Caradoc and Bala felsites, Llandeilo and Arenig Series, frc ; then 

 the Cambrian, embracing the Mira and St. Andrews' Channel 

 series, under consideration at present, and considered by Dr. 

 Dawson as representing the Lingula flags of England. Then 

 the Acadian series of St. John and the Atlantic gold-bearing 

 rocks of Nova Scotia, followed by Basal Cambrian rocks ob- 

 served in New Brunswick, but not yet recognized in Nov^a Scotia. 



Then come the Huronian, considered as represented in Nova 

 Scotia by certain rocks in Yarmouth County, and parts of the 

 districts in Cape Breton mapped by the officers of the Geological 

 Survey as Pre-Cambrian and Laurentian. 



Fossils occur at numerous localities in these measures, and no 

 doubt as they are more fully examined a very complete and 

 characteristic horizon will be extablished. 



