256 PROCEEDINGS OF THE AMERICAN ACADEMY 



the Primordial faiinoe are composed of species having a very limited 

 geographic distribution ; and it is very rare — ahhough the forms are 

 similar — tliat one species extends from one country to another, being 

 common to two Taconic deposits, even when they are very near eacii 

 other. This fact is worthy of notice by all who study the evolution of 

 living beings, tliat at the Taconic epoch the murine zoological provinces 

 were more numerous, and mucli more limited in their geogra])hical 

 extension, and in the number of species and individuals, than in any 

 other geologic epoch, including even that of the present day. 



Lithologically, the greater part of the "Taconic System" is formed 

 of slates, with quartzites here and there, and especially limestone len- 

 tils enclosed in the schists. The size of these lentils varies from that of 

 a kidney or a man's liead to dimensions of twenty, fifty, one hundred, 

 and one thousand feet, and even more ; they are scattered throughout 

 the system, though more persistent and of larger size in tlie upper part. 



^Yith very few exceptions, nearly all tlie Taconic limestones are 

 magnesian. Sometimes they are true dolomites, and contain no fos- 

 sils. But if a very little magnesia is present, fossils are found ; and 

 they are more abundant when the magnesia is in very small quantity. 

 Lastly, if the limestones do not contain any magnesia, and are clayey, 

 then the fossils become directly more numerous, and it is in these 

 lentils of pure lime tluft the Colonies of the second fauna are found. 



Almost all observers who have studied and sought out the pri- 

 mordial fossils in situ have been struck by the total absence, or at 

 least the very great rarity, of fossils in the slates enclosing the lime- 

 stone fossiliferous lentils. AVe have the riirht to conclude that lime 

 was a most important element in the existence of these marine crea- 

 tures of the first period of life on the earth. 



The " Taconic System " is of great thickness, with the exception of 

 Scandinavia, where all the palaeozoic strata are also of slight depth. 

 In America it may be estimated at an average of ten thousand feet. 



