SERPENTINE AT EASTON 421 



of carbonates over into silicates of lime and magnesia, and it 

 is not impossible that the overlying post-Algonkian dolomites 

 may have played some part in the formation of these tremolite 

 beds. This process of transformation was doubtless greatly facili- 

 tated by dynamical forces resulting from faulting, as will appear 

 later. 



Into these, what we may term altered pre-Cambrian sediments, 

 viz., the gneisses with their interstratified bed of carbonates, have 

 been intruded in this limited area at least two kinds of eruptives. 



The first and by far the most abundant is a coarse hornblende 

 granite, w^hich occurs in the form of lenses and bosses, the 

 principal outcrop of which lies on the southeastern slope of 

 Chestnut Hill just across the Delaware River from the south- 

 ernmost extremity of Marble Mountain. It varies from a very 

 coarse to a fine grained rock and is for the most part highly 

 feldspathic, consisting almost wholly of orthoclase but containing 

 vaiying amounts of quartz and an occasional blade of dark green 

 hornblende. In the contact zone the hornblende predominates, 

 replacing locally nearly all the quartz and feldspar. 



The other eruptive, an augite syenite, occurs in much smaller 

 quantities but presents some points of interest which had best be 

 enumerated here. It appears as a much altered type in the 

 serpentine-tremolite rocks, to be described later, lying in the 

 northern part of the granite area above described. It appears 

 again in the same manner in a series of dykes cutting across the 

 beds of gneiss at the northern end of the Bushkill cut. Its 

 principal occurrence, however, is at a point one-half mile east 

 of Walter's Station on the Bushkill Creek, where some six dis- 

 tinct outcrops appear along an obscure fault (marked M. N.) 

 which has the same trend as the gneiss ridge south of it. It oc- 

 curs in the post-Algonkian dolomites and may have been in- 

 truded in the form of a dyke along which slipping subsequently 

 took place, or it may have been faulted up from below. 



Macroscopically the rock consists of an abundance of dark 

 green augite with very little feldspar which gives the rock a very 

 basic appearance. It is for the most part fine grained but is oc- 

 casionally quite coarsely granular, almost granitoid in texture 



