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GEOLOGY AND GEOGRAPHY. 5 lain from place to place, wherever the crystalline rocks sank below the present level, by equally compressed synclinals of limestone. (4) There is a break at the trap dyke of West Conshohocken, disturbing the measures, to an extent not entirely known. Finally there follows a series of tightly folded plications, in which the liydro-mica-schist seems, as a mass, to be always below the lime stone : and finally the unconformable contact with the clays of the Buck Ridge. The conclusion that the writer draws from this small area is also strictly conformable to the conclusions he has drawn from his study of the contact line of limestone and schist from TredyfFrin townships to Quarryville, and in fact along the entire border line of schist and limestone in Chester, Lancaster, York, Cumberland and Franklin Counties, Pennsylvania. So far as the Chester Valley is concerned it adds additional ground for believing the theory established by the writer in his Thesis presented to the University of France in 1882; i.e., that the Chester Valley represents the northern half of a synclinal of which the southern half has been cut off and thrown up on the south, bringing the pre-Cambrian schists in contact with the lime stone.
