MAKING THE ROCK BOTTOM. 



21 



granite had been hardened and pushed part way up, there 

 was a belching of lava from a vent near what is now our 

 northern water line. 



A part of the stuff that came up forms now the island 

 called Black Rock. It is porphyrite, a dark colored rock 

 with white crystals of feldspar scattered through it. All 

 along that northern shore there are cracks filled with the 

 same lava called porphyrite dikes. Seventeen of them can 

 be counted between Green Hill Beach and Pleasant 



Photo, Octuvius Kearny. 



Great Diabase Dike, Jerusalem Road at Cold Spring. 

 A vein of lava thirty feet thick. 1 he lines show the wails of the dike. 



Beach. The thinnest of them measures about one foot in 

 width. 



All these dikes are worn off flush with the granite, and 

 nobody can guess how much higher the lava had to flow 

 before it reached the surface as that surface was when the 

 lava belched forth ; furthermore no one knows how much 

 of it spread thickly over the top. 



