196 TKANSACTIONS OF THE [1892 



in position, we see a huge pile of deposits, chiefly com- 

 posed of dense clays. These formed originally an un- 

 broken aggregate of strata stretching ten to twelve miles 

 back from present tidewater, iitting into the hollows of the 

 Piedmont broken crystalline rocks and rising in a few 

 places forty or more feet over their protuberant surfaces. 

 After the deposition of this highly sedimentary series a 

 period of great activity ensued, which covered large areas 

 of this formation with sand, bi-oke up extensive portions of 

 its mass almost to the base, redeposited in strata large 

 tracts of its surface, and left it eroded, trenched, |)laned oif 

 and covered in many places by cross-bedded sands. The 

 whole area forms a belt not more than fifteen miles wide, 

 running from the northeast to the southwest and extend- 

 ing into Yirginia southwest of the city of Washington. It 

 is to be noted that in Maryland it includes two sets of iron 

 ore clays, the upper of the two being more sandy, and the 

 ore therein formed in nodules and of poorer quality than in 

 the denser clays beneath. 



After the completion of the formation, a period of 

 erosion followed which left deep hollows between many of 

 the hills of clay and along the seaward border especially, 

 carved the hills into low domes, and occasionally stirred 

 the surface into sandy hillocks. 



Albirupean Formation. — Over the top of the broken 

 Potomac came the more sandy mixtures of the Albirupean, 

 filling out the hollows, loading the trenches, and, spreading 

 over the summits of some of the clay hills, kept on away 

 back in tongue-like expansions beyond the inner limits of 

 its predecessor. This is especially the sand-built unit of 

 the subcretaceous, non-marine Epoch. This statement must 

 not, however, obscure the important fact that sedimenta- 

 tion alternated with the deposition of the sands and pro- 

 duced clay strata of wide dimensions. It is also important 

 to observe that during the construction of the clay strata 



