J. S. Newberry— Flora of the Great Falls Coal Field. 195 



flora of the Amboy clays and yet we have obtained from them 

 more than one hundred and fifty species. Probably when as 

 much time shall have been given to the collection of plants 

 from the Amboy clays as has been devoted to the collection of 

 Potomac plants, the number of species will be as large, and 

 better comparisons can then be made between the two floras, 

 but it is evident that they are widely different. From the 

 Amboy clays we have now taken about one hundred and fifty 

 species of plants ; of these more than one hundred, or a large 

 majority, are angiosperms, whereas of three hundred and sev- 

 enty-five species taken from the Potomac group only eighty 

 are dicotyledonous. Besides this, it is doubtful whether any 

 species yet found is common to the two formations. 



The flora of the. Amboy clays is most nearly allied to that of 

 the Dakota group in the far west and the Atane group of 

 Greenland, while one or two species are apparently identical 

 with some taken from the Kome or Lower Cretaceous group. 

 We may therefore fix the horizon of the Amboy clays with 

 absolute certainty at Middle Cretaceous. With equal certainty 

 we can assert that the Potomac, the Kootanie and the Kome 

 groups represent perhaps distinct but closely related epochs of 

 the Neocomian or Lower Cretaceous of the Old World. 



As these determinations have for the most part been made 

 from fossil plants, we must wait for the discovery of plants in 

 the Cretaceous beds of Queen Charlotte's Island and the Shasta 

 group of California before we can accurately correlate them 

 with the Lower Cretaceous strata of Central North America. 

 For this region the history of the Cretaceous age can already 

 be written with a good degree of fullness and its more impor- 

 tant incidents are as follows. 



During the first half of the Cretaceous age the greater part of 

 the continent of North America was out of water and there- 

 fore suffering erosion and receiving no deposition. During 

 this interval a broad, circumscribed and almost inland sea occu- 

 pied the place of the Gulf of Mexico, and the adjoining shores 

 of South America, Mexico and Texas. In this sea marine depos- 

 its were forming which are the equivalents of the Lower 

 Greensand or Neocomian. In time they attained in Chihuahua 

 a thickness of not less than 4,000 feet and represent at least 

 one-half of the Cretaceous age. During this time the area of 

 the plains was out of water and toward the north bore on its 

 surface lakes and marshes where the Great Falls and Kootanie 

 groups were deposited. Beds of coal of considerable thick- 

 ness and now of great importance were formed in these 

 marshes. Up to the present time we have gathered thirty or 

 forty species of the plants which from their debris formed the 



Am. Jour. Scl— Third Series, Yol. XLI, No. 243.— March, 1891. 

 13 



