420 PFXK 



bonates of varying thickness (5 feet to 30 feet) consisting usu- 

 ally of a mixture of pink calcite and gray dolomite but occa- 

 sionally consisting wholly of either the one or the other. . 

 Hornblende (usually tremolite), phlogopite, and occasionally 



y 



Fig, 4. Section No. I, taken along Bushkill Creek. (9 = Post-Algonkian dolo- 

 mites'; 7"= Tremolite beds presumably altered form of C which =1 calcite-dolomite 

 beds ; Gn = pre-Cambrian gneiss. 



pyroxene occur so abundantly in these beds as locally to replace 

 all or nearly all the carbonates, so that locally they become al- 

 tered to a nearly pure tremolite rock with streaks or patches of 

 phlogopite or pyroxene or containing at time considerable 

 amounts of the original carbonates. These tremolite beds are 

 confined to the southeastern slope of Chestnut Hill, and at the 

 southwestern extremity of the ridge lie for the most part at or 

 near the contact of the post-Algonkian dolomites on the pre- 

 Cambrian gneisses. Here, as can be seen in the cut made by 

 the Bushkill Creek, all three, viz., the dolomites, the tremolite 

 beds and the pre-Cambrian gneisses dip uniformly to the south- 

 east and are apparently conformable. (See Fig. 4.) 



The explanation of the occurrence here of beds of tremolite 

 would seem to be simple. In the first place the post-Algonkian 

 dolomites are more or less silicious as the following analysis 

 shows : 



SiO, =3-52 



Fe and Al = 2.92 



CaCO^ =53-40 



MgCOj =40.54 100.38 



Immediately below them come the beds of carbonates which are 

 usually highly dolomitic and may have contained original silica. 

 Underlying these in turn are the pre-Cambrian gneisses which 

 with their included quartz lenses and granite intrusions could 

 furnish free quartz enough by infiltration to transform the beds 



