1884.] 691 [Frazer. 
Trap Dykes in the Archean Rocks of Southeastern Pennsylwoaniu. 
By Dr. Persifor Frazer, 
(Read before the American Philosophical Society, October 17, 1884.) 
Among the geological papers announced to be read in Section E of the 
late meeting of the American Association for the Advancement of Science, 
in Philadelphia, was one by Prof. Henry Carvill Lewis on a Trap dyke in 
Eastern Pennsylvania. It describes a dyke which (its author asserted) 
had been overlooked by the speaker and other geologists in this portion of 
the State, and which was distinguished, both by its great length and by 
certain peculiarities of position,* from other dykes in Pennsylvania. 
This faulted dyke is supposed to have been laterally thrown for a dis- 
tance that was understood to be five miles as Prof. Lewis described it. 
But on hearing that the ‘“hade’’ or dips were nearly vertical in both 
parts which it was thought were once in contact, Prof. James Geikie 
thought that any previous continuity of the two dykes must be aban- 
doned. + 4 
In describing the course of his dyke, Prof. Lewis remarked, ‘Dr, 
Frazer failed to trace it through: Chester county, though he has a small 
portion of it on his geological map in Easttown township ; nevertheless I 
have followed it over the surface, foot by foot, by the loose boulders on 
the surface ; and found it to be continuous,’’ or words to this effect. 
There happened to be on the wall the joined maps of the four counties 
which the speaker had prepared for the Second Geological Survey, viz: 
Adams, York, Lancaster and Chester, and he referred to them as follows: { 
The great amount of disintegration of the surface rocks of Chester 
county has caused a deep soil, which overlies a large portion of the rocks 
of the county on the line of this dyke, and the constant movement of this 
soil renders it very difficult to trace the buried outcrops by loose boulders 
and fragments. The consequence of this is that if one maps all the locali- 
ties where masses of trap are found, and attempts to connect them by lines, 
the irregularity of the latter will inform him that he is probably not 
representing the facts of structure as they exist. In Adams county, where 
the decomposition is generally much less profound than in Chester, in the 
* The text of this paper is not at hand and the writer must trust his memory 
for its contents. It is very unfortunate that in the reports of the proceedings 
of Sections of the A, A. A.S., there should be no account taken of the discus- 
sions on papers; especially in cases where statements are observed and 
pointed out which seem to be at variance with a cautious judgment of the 
facts, The disadvantage of this state of things to the cause to which the 
Association is nominally devoted is still further increased by the long period 
which must elapse before a paper finds its way into print. Error is notoriously 
fleet of foot, and with a year’s start may defy pursuit. 
+ In a rather exceptionally full notice of this paper (omitting however men- 
tion of the objections to it) given in Science subsequently, the fault is stated to 
pe several thousand feet, but the extent of the lateral displacement is men- 
tioned only as “large.’’ 
{The following remarks are quoted from memory and somewhat amplified, 
