Maryland Geological Survey 149 



factory evidence of angiosperms in beds which are classed as Aptian, and 

 by the close of the Albian dicotyledons become a considerable element in 

 the floras. As regards species they form 30 per cent of the Patapsco 

 flora, 17 per cent of the Fiison flora, and over 35 per cent of the Albian 

 flora of Portugal. 



It would doubtless be interesting to pursue the subject in more detail, 

 to discuss the probable place and manner of origin of this latest and most 

 highly organized class of plants, as well as its early paths of migration, 

 but the subject is so highly speculative that it may well await the en- 

 largement of the bounds of our knowledge. 



The same statement is in a measure true of- attempts to describe Lower 

 Cretaceous climatic conditions. The floras are so different in every way 

 from those of the present that it is unsafe to lean too strongly on the 

 facts which may be deduced from the present climatic distribution of 

 the sometimes remotely related representatives of these ancient types in 

 the existing flora. It would be foolhardy to guess at actual temperatures. 

 With regard to more general statements and questions of moisture the 

 basis is not quite so insecure. The floras certainly show such slight 

 changes which may be legitimately related to temperatures when they are 

 traced from place to place, that a marked uniformity of temperature 

 conditions over a great man}'- degrees of latitude must be admitted. The 

 accompanying sketch map shows in a general way the location of the 

 known Lower Cretaceous floras. That from Peru is within 15 degrees 

 of the equator, while that from western Greenland is in latitude 70° 

 North, and that from Spitzbergen is from latitude 78° North, and even 

 if the latter is latest Jurassic instead of earliest Cretaceous, it serves 

 equally well to indicate that climatic conditions were much more uniform 

 than they are at the present time. 



It seems quite obvious from a consideration of the large-fronded ferns 

 and cycads of the Potomac flora that they could not have withstood a 

 winter as severe as the average winter of to-day in the latitude of Mary- 

 land. The petrified woods show seasonal changes, but the width of the 

 active growth-ring is very wide, and that of restricted growth is narrow 

 and more or less irregular, and is as readily explained by a dry season as 



