INTRODUCTION 189 



The remains of conifers are considerably more common than those of 

 other types, though such may not have been actually the case during 

 that period, as the trunks of the former are much better adapted for 

 preservation than most of the other forms. 



The most variable element of the flora is the dicotyledonous. In the 

 lower beds the forms are scant, as well as primitive in type. In the suc- 

 ceeding deposits they become progressively more and more specialized 

 and abundant, until, in the, uppermost beds, there is a wide range of 

 highly organized genera and species with well marked modern affinities 

 and a great profusion of individuals. 



The fauna includes sponges, either worm or insect larvae borings, insects, 

 lamellibranchs, gastropods, fishes, and reptiles, including plesiosauria 

 and dinosauria. Remains of the last mentioned group, including both 

 diminutive and gigantic species, are by far the most important, and serve 

 to confirm the evidence of the cycads as to the Mesozoic age of the de- 

 posits, as well as of the prevailing warm climate. The absence of any 

 strictly marine fossils* and sediments, together with the presence of a 

 few brackish water shells, point to estuarine conditions of deposition. 



The sands and clays have been largely drawn upon for building and 

 other purposes, and this fact, together with the alternately argillaceous 

 and arenaceous character of its soils, early gave rise to the name "clay 

 and sand belt." 



The name " Potomac " was first applied to the lower and middle por- 

 tions of these deposits by Professor W J McGee,t of the U. S. Geological 

 Survey, who began his studies in the Potomac River basin near Wash- 

 ington, D. C. 



It hardly needs mention here that the Potomac deposits have been 

 the subject of a great amount of study by many independent workers, 

 who have approached the problem from nearly as many different points 

 of view. These facts, together with the proverbially complicated stratig- 

 raphy, have given rise to a highly varied taxonomy and nomenclature 

 and a corresponding amount of not always the best-humored contro- 

 versy. The views of the several writers, including those of the authors 

 of this paper, as set forth in the Journal of Geology in 1897, are shown 

 on the accompanying comparative taxonomic table. 



It is believed by the authors that the Maryland section contains more 

 facts on which to base a solution of Potomac problems than any other. 

 To begin with, it is centrally located, well within the belt. A study of 

 the margins of sedimentary formations is apt to convey erroneous im- 



* Spicules of sponges are often common in clearly defined estuarine deposits. They occur in 

 abundance in recent estuarine sediments of the Chesapeake as far north as Baltimore, 

 t Report of health officer, D. C, 1884-'5 (1886), p. 20. 



