DIFFUSION OF THE CAMBRIAN FAUNAS. 



13 



VIT. — Macltirf.a— Tetraorapttis. 



CnAZY — Arbnki fauna. 



REFERENCE. 



Areas of crystalline and nietamori)liic rocks, supposed to have been above the .sea in Canibrian 

 times. 



;■ ' 1 Ontcrops of the Chazy fauna, and of the sandstones .snppased 10 bo of that ai:e. The south-westward 

 extension of this fauna is not indicated. 



Outcrops of the Arenig fauna. The soutli-western extension of this fauna is not shown. The fauna 

 is said to have been recognized in Kast Tennessee by trilobites (but apparently not by grap- 

 tolites). Outcrops are reported from Idaho and from British Columbia, beyond the limit of 

 the map. Compare the range of the two faunas represented on this sketch map with that of 

 Olenellus and Paradoxides respectively on the other map at page 9. The Dicellocephalus- 

 Dictyonenia faunas, intermediate between the two sets of faunas repre.sented on these maps, 

 show a similar relative geographical range. 



The Grulf of St. Lawrence seems to have been at more than one period in the past as 

 it is now, the resting place of faunas, which had been driven out of other regions. At 

 the present tinae there exists there an isolated remnant of the Virginian fauna. This was 

 made known many years ago by Sir Wm. Dawson, and more recently Prof. W. F. Granong 

 of Harvard, has found many sovithern forms not previously recorded from the Gulf 



At an earlier date this Gulf atforded a refuge for the last remnants of the Ordovi- 

 cian fauna, driven out of the region of New England and New York by the physical dis- 

 turbances which occurred there at the close of the Ordovician age. 



At a still earlier time a great development of the closing phase of the Upper Cam- 

 brian fauna took place in the northern part of this Gulf The limestones which contain 

 this fauna are found chiefly near the Strait of Belle Isle. These limestones and included 

 shales show a regular passage from the Cambrian to the Ordovician fauna, which fauna 

 is delimited by the appearance of Phyllograptus of the Quebec Gh-oup. 



We owe to Mr. Billings a description of the fauna which filled these IIppi-i- Cam- 

 brian limestones, and he has also given his judgment on the equivalency of these beds with 

 certain strata in the state of New York and in Europe. 



These limestones and the associated beds are aboiit 8,600 feet thick, and have been 

 divided into four principal sections, based on the contained faunas. The " Lower Pots- 



