Lewis.] '^•^^ [May 15, 



Centre Hill, where it is clearly marked by numerous boulders. From 

 here to the "Meeting House," near the fault, it was traced by Judge R. 

 Watson of Doylestown, who reports it to be continuous, and Mr. Pas- 

 chall states that it abuts against the fault. 



This dyke is joined by another one, of entirely diflferent structure, near 

 Gardenville. This latter dyke, composed of a coarse-grained, almost 

 granitic diabase, runs in a straight line from here to Point Pleasant, where 

 it forms a large outburst and is finely exposed on the river bank. This 

 dyke is here much wider and more massive than the long dyke which we 

 have been following, and the diabase is not so hard nor so resonant. The 

 decomposition products are also dissimilar. Yet both are true diabases, 

 diflering mainly in their degree of coarseness, and, as the following analy- 

 ses show, of very similiar composition. Dr. F. A. Genth* has analyzed 

 the diabase at Point Pleasant, and Mr. F. A. Genth, -Jr., has analyzed 

 that from Gulf Mills, Upper Merion township, Montgomery county. The 

 latter is typical of the diabase of the long dyke. Analysis No. I is of the 

 Point Pleasant diabase. No. II of the Gulf Mills diabase. 



(I) (ir) 



Loss by ignition 0.29 2.15 



Silicic acid 52.91 51.56 



Titanic acid 1.03 1.63 



Phosphoric acid 0.12 0.13 



Alumina 14.45 17.38 



Ferric oxide 0.42 6.57 



Ferrous oxide 4.S8 3.85 



Magnesia 8.34 3.42 



Lime... 9.92 10.19 



Lilhia trace. 



Soda 1.80 2.19 



Potash 0.57 1.46 



100.23 100.53 



Of similar structure are two other large outbursts of trap which form 

 " Solebury mountain " and "Jericho hill " in Solebury and Upper Make- 

 field townships respectively. These, like the Point Pleasant dyke, appear 

 to have no connection with the long narrow dyke which, with its contin- 

 uation north of the great fault, forms the subject of this paper. They are 

 composed of a light colored, coarse-grained diabase, crumbling when 

 decomposed, and quite unlike the dense dark rock of the long dyke. 

 These dykes have the curved shape so often seen in the ordinary Triassic 

 dykes, of which those in the Connecticut valley may be taken as types. 



While our long dyke represents a simple crack in the earth's crust, 

 the strata on either side being unaltered, these crescentic dykes are accom- 

 panied by disturbances in the surrounding strata. Thus the Triassic shales 



* Report C', p. 18i. 



