114: Marsh — Jurassic Formation on the Atlantic Coast. 



Following these well known writers, others of less expe- 

 rience in the West have repeated their statements or followed 

 the earlier geologists as to the age of western horizons, and 

 thus tended to continue the confusion where the facts them- 

 selves made the whole subject clear. Thus it has come to pass 

 that while the Jurassic formation has been recognized in the 

 Arctic regions of this Continent, and along the Pacific coast, 

 especially through Oregon and California, as well as in Mexico, 

 and likewise in various parts of South America, its develop- 

 ment in the Rocky Mountain region has received little atten- 

 tion except from those especially engaged in its investigation. 

 It is not strange, then, that those who have not seen how 

 extensive the Jurassic formation is developed in Europe, and 

 have not examined its characteristic exposures in the West, 

 should fail to recognize it on the Atlantic coast where its 

 features at many points are obscure. 



In my paper on this subject last year, I endeavored to 

 show that the burden of proof must rest upon those who 

 denied the existence of the Jurassic formation on the Atlantic 

 border. The evidence against it is still based mainly upon 

 fragmentary fossil plants, in regard to the nature of which the 

 paleobotanists themselves are not in accord. 



Cycad Horizons. 



I have recorded elsewhere my opinion of the comparative 

 value of different kinds of fossils, — vertebrates, invertebrates, 

 and plants, — as evidence of geological age, and have endeavored 

 to show that plants, as usually preserved and described, are the 

 least valuable witnesses. The evidence of detached fossil 

 leaves and other fragments of foliage that may have been car- 

 ried hundreds of miles by wind and stream, or swept down to 

 the sea-level from the lofty mountains where they grew, should 

 have but little weight in determining the age of the special 

 strata in which they are imbedded, and failure to recognize 

 this fact has led to many erroneous opinions in regard to 

 geological time. There are, however, fossil plants that are 

 more reliable witnesses as to the period in which they lived. 

 Those found on the spot where they grew, with their most 

 characteristic parts preserved, may furnish important evidence 

 as to their own nature and geological age. Characteristic 

 examples are found among the plants of the Coal Measures, in 

 the Cycads of Mesozoic strata, and in the fossil forests of 

 Tertiary and more recent deposits. 



As bearing directly on the question here discussed, the 

 Cycads of the Jurassic period afford instructive examples of 

 the evidence that may be derived from fossil plants under 

 favorable circumstances. The Cycad trunks of the upper 

 Jurassic of England have long been known, and are especially 



