296 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



Iii the neighborhood of the intrusive gabbro mass the Baltimore gneiss 

 loses its stratified character and becomes massive and darker colored. The 

 change in color is due to the development of hornblende, or more rarely 

 augite, and of garnets, which develop in great profusion in the contact 

 zone. The central body of gneiss is very completely penetrated by the 

 gabbro. The banded structure is therefore much more conspicuous on 

 the flanks of Buck ridge. A massive granitic character is considered an 

 indication of the proximity of gabbro, although the gabbro may not 

 always be exposed at the surface. 



Thickness, correlation, and name. — There is no means of estimating the 

 thickness of this formation, which is the floor upon which the other 

 members of the sedimentary series were laid down. This gneiss under- 

 lies material known to be of Cambrian age and presumably Ge< rgian. 

 It is therefore pre-Cambrian and is correlated with the pre-Cambrian 

 Stamford gneiss of western New England, with the Fordham g T eiss of 

 New York state, with the Baltimore gneiss of Maryland, and the Carolina 

 gneiss of the District of Columbia and Virginia. While the pre-Cambrian 

 gneiss of Pennsylvania is not stratigraphically continuous with the 

 Baltimore gneiss of Maryland, similar stratigraphic relations, like litho- 

 logic character, and proximity of the two formations have found recog- 

 nition in a common name. The pre-Cambrian gneiss of Maryland has 

 been called the Baltimore gneiss because of a fine exposure of it on Jones 

 falls, in the city of Baltimore. This name is also given to the pre-Cam- 

 brian gneiss of Pennsylvania. The Baltimore gneiss includes H. D. 

 Rogers' " primal lower slate " and a part of his northern or " third gneiss 

 belt." The major part of the " third gneiss belt " is gabbro. Both the 

 Baltimore gneiss and the gabbro are included by the Second Geological 

 Survey of Pennsylvania under the term " Lauren tian gneiss.'" 



CAMBRIAN ROCKS: CHICKIES QUARTZITE 



Distribution. — This hard resistant formation constitutes the north 

 Chester Valley hills, the highland at Hickorytown and Coldpoint, the 

 hills west of Whitemarsh, Fort hill, Camp hill, the highland about Wil- 

 low. Grove, and the long ridge known as Edge hill and Lafayette hill ; also 

 the north Huntington Valley hills. 



Character of the formation. — If possesses a conglomeratic lower mem- 

 ber, which is largely composed of elongated pebbles of the blue quartz 

 which characterizes the Baltimore gneiss. This member is brought to 

 the surface in the nose of a pitching syncline. It may be found 1 

 mile south of Morganville and an equal distance east of Willow Grove. 

 It is also brought to the surface on the middle limb of a fold at the base 

 of the north Chester Valley hills, near the dam at Valley Forge. This 



