MAKING THE ROCK BOTTOM. II 



that the solid rock surface is nowhere in town much farther 

 below our daily vision than this in the Picle. 



The highest point of solid rock is in the southwestern 

 part of the town, near the Hingham line, not far from 

 Doane Street. There the height of about one hundred 

 and thirty feet abov^e sea level is attained by the uneven 

 granite floor. From thirty-three feet below to one hun- 

 dred and thirty feet above the mean water line gives a 

 maximum undulation of one hundred and sixty-three feet. 

 Assuming the town to be two and a half miles wide, the 

 greatest unevenness would be only one eightieth of the 

 breadth of the town. 



It will be readily seen, therefore, that the rock bottom, 

 in spite of all its ledges and its channels, comes propor- 

 tionately much nearer being smooth than an ordinary door- 

 step or the palm of one's hand. 



But, smooth or rough, according to the standard of es- 

 timate assumed, the course of events which made it what 

 it is may be partially narrated. 



It will be noticed that the rock is granite at almost 

 every point where it crops out, and granite is a kind of 

 rock which cannot be formed except under immense pres- 

 sure and in the presence of confined moisture. If a piece 

 of it were to be melted in a crucible it could never become 

 granite again upon cooling, but only a glassy lump of 

 slag ; for it needs a weight of more than fifteen thousand 

 pounds upon every inch of it, and also it needs a great deal 

 of moisture for the crystals to form as they are in granite. 

 Furthermore, these same quartz crystals take a very dif- 

 ferent shape when free from pressure. 



In the granite they have accommodated themselves to 

 the spaces left around the black crystals of hornblende 

 and the milky crystals of feldspar. 



Both the hornblende and the feldspar seem to have got 

 into shape first ; whereas in the ordinary conditions above 

 ground, quartz crystals harden sooner than these others. 



