MAKING THE ROCK BOTTOM. 2/ 



But more than any of these, slants the familiar dike on 

 Little White Head. From thirty to forty-five degrees 

 away from the vertical, this vein of diabase hades towards 

 the southeast. The extension downwards may change in 

 direction in some cases, but in most of our dikes it is 

 reasonable to suppose a straight course downward to the 

 place whence the lava issued. The stuff of which this 

 diabase, the youngest of all our solid rock, is made is 

 nearly the same as that which made the diorite, the oldest 

 of all our solid rock. It is principally triclinic feldspar, 

 not the milky orthoclase, but the dark oligoclase. 



The similarity between this lava that came up last from 

 beneath the granite and the diorite which formed first 

 above the granite leads us to suspect that at one time the 

 granite ceased forming for a time, say millions of years, 

 beneath Cohasset ; and that the molten stuff rearranged 

 itself again as before, with the triclinic feldspar and other 

 basic minerals upon the outside, and the quartz or acid 

 stuff beneath. 



So it happened that the feldspar when it was ready to 

 harden into a second layer of diorite was heaved up 

 through all the granite and through the first diorite, 

 becoming dikes of diabase, and possibly overflowing on 

 top in great beds of diabase. 



Subsequently the overlying beds of rock were worn off, 

 and nothing is to be seen now but the narrow vertical 

 vents hardened into dikes. What could have made the 

 granite stop forming and at the same time could have 

 stirred up a new mixture of molten stuff so that the feld- 

 spar must gather as before at the outer part of it can be 

 fairly conjectured. Waves of heat move no doubt from 

 place to place within the earth, and remeltings with mix- 

 ings must be continually taking place ; but the dikes are 

 lying silent before any one who will hazard a guess. 



Besides the triclinic feldspar there are many other min- 

 erals in small proportions within the diabase. Epidote 



