1878.] 1^1 [Lesley. 



and this must be taken as the best expression of the chemical distinction 

 between the purer and the more magnesian limestone layers of our Siluro- 

 Cambrian (Calciferous-Sandstone) Formation, No. II, which we can 

 make at present. 



It shows plainly enough that the magnesian limestones are very far 

 from being typical dolomites. 



It shows also that the presence of magnesia at the expense of lime is 

 connected normally with a high percentage of alumina silicate. 



This, it seems to me, goes one step towards settling the mechanical theory 

 of the origin of the magnesian carbonate on a sound basis. Although we 

 may have to seek long for the source of the sediment, as a whole it must 

 have a source which is common also to the clay. 



Difficulties multiply upon us in studying such data. No satisfactory ex- 

 planation of the bedplate structure of the mass has yet been offered. If 

 the deposit be in the main mechanical and not chemical, it is strange that 

 such sharp distinctions between layer and layer should have been made in 

 the bed of a deep ocean. It is still more strange, that Con this hypothesis) 

 strongty marked local abnormal analyses should be encountered. 



This leads me to say that the above investigation is imperfect because 

 carried on in a vertical plane only. It should now be repeated in a hori- 

 zontal plane. It is desirable to learn whether the geographical changes 

 may not be great enough to convert a limestone bed here into a dolomitoid 

 there, half a mile (or perhaps 100 yards) distant. If this prove true (and 

 the possibility of it is indicated by the abnormal analyses), then a new 

 difficulty arises in the way of a sound theory of the origin of the bed plates ; 

 and confuses still more any mechanical theory of the sediments. 



Finally, it is evident from Table 4, that if we take 50 beds together and 

 compare them with the 50 lying next beneath them, in other words, when 

 we compare together two masses of the formation one or two hundred feet 

 thick, — it is evident that, in one long age of deposit, more clay and magnesia 

 were present in the ocean than in the preceding (or succeeding) long age. 



We have then a large curve of variation, including many small curves, 

 much more strongly marked than in the large one ; like the monthly curves 

 superposed upon the annual path of the moon. 



Were it not reckless to hazard a suggestion that the source of the mag- 

 nesian element is to be sought for in some theory of the ejection and distri- 

 bution of volcanic dust, so that each short time of volcanic disturbance 



PROC. AMER. PHILOS. SOC. XVIII. 103. P. PRINTED FEB. 25, 1879. 



