52 



ments. This is owing partly to their being of less volume originally, 

 for they cannot be estimated at more than 500 to 700 feet in any of 

 the three basins where they are so exposed that an approximate esti- 

 mate can be made. But probably the chief reasons of the rarity of 

 exposures of this part of the St. John terrane, are the softness of the 

 slates or shales of which the division is composed, and to the fact that 

 they form the uppermost division of the terrane, and therefore were 

 the first to come under the destructive action of abrading agencies. 



Approximate A more careful examination of the field is necessary before a reli- 



thickness of . _ j ^ 



the Cambrian able estimate could be given of the thickness of the Cambrian rocks 

 in Cape Breton, and the following are to be regarded as only an ap- 

 proximate estimate of their thickness. 



East Bay Mira Valley. 



Coldbrook terrane .... ...... 300 Very thick. 



Etcheminian terrane , 500 3,000 ? 



St. John terrane, viz : 



Acadian division . .'. . 200 800 ? 



Johannian division 1,200 2,000 1 



Bretonian division 500 ? 500 ? 



2,700 ? 



ORIENTATION. 



Having observed a remarkable uniformity in the attitude of the valves 

 of Brachiopods buried in the Etcheminian sediments of Indian Brook 

 basin on the East Bay of the Bras d'Or, it occurred to the writer that 

 an investigation of the causes that led to this would throw light on the 

 physical history of the Cambrian deposit in relation to the life of that 

 period. 



Attitude of I n the following remarks I shall use the term orientation to express 

 the valves of. .,. ,.,, , P , , i i < i i 



the Brachio- the attitude in which the valves of the brachipods are round, when 



&*** opened up on the layers of the rock over which they are spread. The 



beak of the ventral valves is found to point so uniformly in one 

 direction that it can only be the result of some general cause which 

 has acted on these valves when living, or when about to be buried in 

 the mud on the sea-bottom. 



These brachiopods when living would have been attached to the 

 bottom by the pedicle or anchoring thread, as in the modern Lingula, 

 but were free thus to float in the sea water near the bottom. The 

 great majority of the shells buried in the Etcheminian sands and 



