Maech 12, 1897.] 



SCIENCE. 



413 



are found in the Potomac. The Ehetic is 

 also penetrated by numerous dikes of igne- 

 ous rock, none of which pass into the Po- 

 tomac beds. The interval of time sepa- 

 rating the two formations must, then, have 

 been long enough to permit the occurrence 

 of important geologic changes. These re- 

 sulted in the di-aining off of the Ehetic 

 waters and in the lateral compression of 

 the Ehetic areas, which caused crumpling 

 and faulting of the strata and outpours of 

 igneous rock. The Ehetic basins were 

 elevated and formed into dryland, subjected 

 to great erosion, and the most eastern of 

 them then depressed and brought under 

 water again. Certainly no Rhetle species of 

 plant survives into the Potomac." 



From all this it is apparent that the Older 

 Mesozoic or Triassic formation of the Atlan- 

 tic border has really nothing to do with the 

 Potomac formation. Only in a few places, 

 as through parts of New Jersey and in Vir- 

 ginia for a short distance in the vicinity of 

 the North Anna and South Anna Eivers, are 

 the two formations in contact, and here the 

 latter rests in complete unconformity upon 

 the former. At all other points they are 

 separated by an interval of greater or less 

 width of the old crystalline rocks. This 

 shows that the Trias, as well below the 

 Hudson as in the Connecticut Valley, con- 

 stitutes a trough and forms no part of the 

 Coastal Plain proper, having its afBnity 

 much more closelj' with the Piedmont 

 Plateau. The fact that not a single Triassic 

 species passes up into the base of the Po- 

 tomac further proves that that interval 

 must have been an exceedingly long one, 

 and it is quite in conformity with the facts 

 to suppose that it embraced the entire Ju- 

 rassic period. 



The little that Professor Fontaine has to 

 say of the relations of the Virginia beds to 

 those of Maryland and farther north is im- 

 portant and shows that, although he had 

 not studied the latter except in a general 



way in Maryland, he had nevertheless 

 formed a tolerably accurate opinion as to 

 their nature. On page 14 he says : 



" It should be stated that there is reason 

 to think that the extensive formation of 

 clay and fine sand known in Maryland as 

 the 'variegated clay formation,' or the 'iron- 

 ore clays,' may belong to the same general 

 epoch as the Potomac of Virginia, forming 

 an upper member of the group of which 

 the Virginia Potomac is the lower. The Vir- 

 ginia beds and those of Maryland cannot now 

 be certainly separated by any sharp differ- 

 ences ; hence, for the present, the Virginia 

 strata must be regarded as Lower Potomac, 

 and the Maryland formation as Upper Po- 

 tomac." And, again, on page 142 he makes 

 the following statement : 



" On entering the District of Columbia 

 two members of the Potomac formation 

 may be recognized. The lower is that 

 traced through Virginia, and this is the 

 only member recognized in that State. 

 From the predominance of sand and sand- 

 stone in this it may be called the sandy 

 member. The other, or upper member is 

 composed of sands and clays, mostly the 

 latter, both being usually highly colored 

 with tints due to oxide of iron. The clays 

 greatly predominate. They have the colors 

 arranged in irregular spots, patches, and 

 seams, and on account of this they have 

 been called by Mr. Philip Tyson and Pro- 

 fessor Eogers the variegated clay group. 

 This is the upper member of the Potomac 

 described at Fort Washington. 



" The sandy lower member of the Potomac is 

 visible at Washington and at several points be- 

 tween Washington and Baltimore, in the vicinity 

 of the Baltimore and Ohio Railroad. The 

 farthest point north at which it has as yet 

 been seen is Baltimore." 



It is clear from this that Professor Fon- 

 taine believed that the Older Potomac ex- 

 isted in Maryland. I remember his saying 

 to me at about the time that I began my 



