Frazer. ] 692 (Oct. 17, 
region of the dykes, a great deal of work was necessary before the seeming- 
ly capricious outcrops could be brought into anything like order. Let any 
one look at the regularity of the three threads of trap passing from N. E. to 
§. W. on the map of Adams county of the First Geological Survey, and 
compare it with the irregular and broken lines of the trapin the map of 
the same county by the present Survey, and he will find a case in point. 
It will not suffice to find three or four occurrences of fragments of trap, 
lying more or less ina straight line, ina distance of a mile or more, in order 
to assume a dyke of trap connecting them under the soil. 
On looking over the maps of the townships south of the Chester valley, 
which the speaker carried into the field when engaged in the Geological 
Survey, he remarks in a great many places notes of trap fragments on the 
surface. But with some experience in tracing the outcrop of this rock, 
he did not feel justified in connecting these isolated indications together, 
and he still doubts whether this should be done. The absence of a map 
of the dyke prevents him from saying how many of these occurrences are 
included within the dyke mentioned by Prof. Lewis, but no single dyke 
can include many of them. 
It is a very different matter if it is merely claimed that this supposed 
new dyke indicates the direction of a zone or belt of disturbance along 
which two, twenty, or fifty outbursts of igneous rock may have taken 
place, just as it is shown that in north-western York and central Adams 
counties, notwithstanding all the irregularity of the outcrops, there is a 
general zone along which the main outflows have taken place. 
The considerations which the speaker has so often urged in connection 
with this region, make the existence of such a belt exceedingly probable, 
(See Mémoire sur la partie 8. E. de la Pennsylvanie, pp. 90, 109 +, ete.) It 
has been abundantly urged, both in this memoire and elsewhere, that the 
exceedingly straight southern limit of the Chester valley implies other 
causes at work than those of ordinary deposition : in other words, a great 
longitudinal crack along the southern side of which the lower measures 
were brought up ; that this great crack would in all probability be connected 
with others crossing or diverging from it hardly needs to be stated; but 
if the speaker was unable to represent this line of fracture by a single 
well defined dyke, there are abundant allusions both to outflows of 
trap and to the existence of a belt of dislocation, as the following from 
O,* will show, p. 286. ‘The trap dyke, traceable only by its broken 
fragments} on the surface, which has been alluded to as occupying the 
southern edge of Tredyffrin township, enters Hasttown,”’ etc. (here follows 
a detaiied statement ofits course), * * * ‘when its traces cease to be 
apparent, though a few scattering boulders and fragments of trap are met 
with.’’f ; 
*Geology of Chester county, Persifor Frazer, edited by J. P. Lesley, 
+Of course, a trap dyke may be assumed when the whole ground is made up of 
the larger or smaller fragments, but the question how many trap fragments will 
enable one to assume the presence of a dyke 1s, like many others, not capable of 
a general answer, 
{The trap here referred to is part of the “great dyke” which forms the sub- 
ject of the paper above alluded to, 
