90 PROCEEDINGS OF THE ACADEMY OF [1898. 



VOLCANIC ROCKS OF MESOZOIC AGE IN PENNSYLVANIA. 

 BY E. GOLDSMITH. 



My petrographical and geological observations have convinced 

 me that nearly all so called ''trap" is of volcanic origin. Sup- 

 porting proofs may be found in many localities near Philadelphia 

 such as the Haycock Mountain, New Hope and the railroad cuts 

 north and south of Quakertown in Bucks County, Pa. The richest 

 field of observation, however, is the hill four miles north of Potts- 

 town, Montgomery County. The various outcrops of the volcanic 

 rocks may there be studied to greater advantage than in the other 

 localities. 



One hundred and fifty acres of the top and side of the hill now 

 form a public park, much of the rubbish and underbrush being, 

 therefore, removed. A series of volcanic terraces of Mesozoic age 

 has been thus uncovered and is, I think, worth recording. These 

 terraces were formed by four distinct outflows of lava. The four, 

 five or six sided columns of greenstone-basalt can be seen on the 

 edge of each step from the first outpour. The first outflow had a 

 ran<re wider in extent than the others ; the second was less, to all 

 present appearances; the third eruption was decidedly less great, 

 whilst the fourth, the top of the hill, exceeded the second and 

 third in mass of material. The greatest difference between the 

 levels of the various outpours is seen between the third and the 

 fourth flow. The whole extent of these terraces is overshadowed 

 by a grove of trees and carpeted with grass and other small plants, 

 an illustration would, therefore, be indistinct. 



A strictly scientific examination of the terraces would involve a 

 survey, the removal of rubbish and sod down to the lava and the 

 determination of the various levels with instruments of precision. 



The terraces are situated on the western side of the park. The 

 level forms a plateau on which a fine iron structure has been built 

 to a height of some twenty or more feet above the trees, an obser- 

 vation tower, from the upper platform of which a grand view of 

 part of the Schuylkill valley can be obtained. Near this point, in 

 a pile of debris, the first indications of a devitrified obsidian or tach- 



