14 GLIDE TO LOCALITIES. 



FINIS HILL, MEDFORD. 



Jtunte.— By rail, Boston and Maine road from North Union station to 

 Medford, five miles. Single fare s.10. By electrics, Medford and Maiden 

 cars from Scollay square to Medford square. 



By vlu'i'l, cross Charlestown bridge east of North Union station, through 

 City square and Sullivan square, Charlestown ; Winter Hill avenue and 

 Mystic avenue to Medford square. 



Continue through Medford square by all routes, north for one-half 

 mile to point where sign " Middlesex Fells Reservation " appears on left. 

 Turn off along driveway, keeping to right at junction of roads. Dike 

 appears here underfoot and for nearly a mile north. 



A dike of diabase, commonly known as the " 300-foot dike," 

 lies in the main between two hills of country rock. Along the 

 road, diabase soil can be contrasted with glacial soil. The age of 

 the erosion can be noted, and the contact on either side followed 

 quite closely . On the west, at the south or nearer end, it is very 

 involved ; and at various places along the margin on both sides 

 other dikes having an east-west strike can be found. The country 

 rock in the southern portion of the field is porphyrite ; in the 

 northern, granitite. The relations of the two are not shown. The 

 former varies considerably in the size and abundance of the ortho- 

 clase crystals. The diabase also varies much in texture from 

 center to sides, becoming in some places a fine-grained trap. It is 

 probable that parts show the microstructure of basalt rather than 

 of diabase. At the north end of the walk, where the dike is lost 

 to sight for some distance, a small section of the east contact in- 

 dicates a slight dip to the westward away from the vertical. What 

 is probably the same dike appears also at the old Powder house in 

 Somerville, and in a quarry on Granite street in the same city. 

 The former shows inclusions of rocks which lie beneath the sedi- 

 ments of the Boston Basin. 



Perhaps the most interesting feature of the dike is the weather- 

 ing. The southernmost quarry exhibits the concentric arrange- 

 ment well, and all stages from compact rock to fine gravel can be 

 seen. Iu any case, however, slight changes have taken place, and 

 the powdered rock effervesces freely with hydrochloric acid. To 

 the south near Medford square, along Governor's avenue, road-cuts 

 also show residual boulders and concentric peeling. Good photo- 

 graphs can be obtained. 



