297 
pected that nearly or quite every specimen known from any part 
of the United States will be included. Prof. F. H. Knowlton will 
be associated with me in the publication of this memoir, and to , 
him will be entrusted the important subject of their internal 
Structure. | 
With regard to the first of the two above-named objects which 
I had in view, I may mention that the Woman’s College has 
afforded me the much-prized opportunity of spending a week in 
the field with Mr. Bibbins in visiting all the localities at which the 
trunks have been found. In most cases, as already remarked, 
they were found by Mr. Bibbins at different places from those in 
which they had been first discovered, but there was always some 
quite definite knowledge to be obtained as to the original locali- 
ties. These were themselves, it is true, often little calculated to 
throw light on their geological position, the earliest history in 
many cases not going back of a time when they were seen in 
Some field or by the roadside in the country. I may here say in 
anticipation of what will be said in the general report now in 
Preparation for publication by the Geological Survey on the 
Seology of the Potomac formation, that in the course of the last 
five or six years, during which time I have been more or less’ 
actively engaged in the study of that formation, I have been able 
with some success to subdivide it into no less than six somewhat 
distinct horizons, and it was therefore directly germane, to my 
investigations that the horizon to which the cycads belong should 
be fixed with certainty. Tyson, as has been seen, reported that 
they were found in the iron ore clays, and it has been the general 
belief ever since that this was their position. I had observed that — 
the large quantities of silicified wood and most of the lignites that 
®ccur in this formation come from a much lower horizon, viz., 
that of the F redericksburg freestone, which in Maryland is never 
hardened into rock, but exists in the form of loose sand and 
Sravel, as indeed it does to a large extent in Virginia. Having 
found Silicified wood immediately associated with the cycadean 
trunks of the Black Hills, and having also learned that it occurs in 
the same beds: in which Professor Cragin found a similar speci- 
men in Southern Kansas, I had suspected the possibility that the : 
‘ame would prove to be the case in Maryland, a view which P ae ok 
