38 WALKS AND TALKS. 



the State House dome, demolished by the giant's wife and 

 her screaming boys, refers to a conglomerate. 



" They flung it over to Roxbury hills, 



They flung it over the plain ; 

 And all over Milton and Dorchester, too, 

 Great lumps of pudding the giants threw ; 



They tumbled as thick as rain." 



Well, here is a rock with shining scale-like mineral frag- 

 ments. Pick up a scale with your knife-blade. Do you see 

 it split into laminse or leaves of indefinite thinness? "Yes," 

 you say; "this is the same thing as is used in the doors of 

 our stoves to permit the light to shine through ; only these are 

 black leaves and those are transparent." Quite right. What 

 do you call the mineral in your stove door? "Mica, though 

 some people call it isinglass." Mica is correct. One species 

 of mica is black, and has a particular name ; another varies 

 from dark-brown or smoky to transparent, and has a different 

 name. There are also some other species of mica. So you 

 know mica. 



Examine this rock very closely do you find any quartz? 

 "Yes," you say, "there are two kinds of light-colored min- 

 erals here." Carefully test them both for hardness. Can you 

 scratch them? " Well, no. One of them is hard enough for 

 quartz it is quartz ; but the other I am uncertain about." 

 Then you must try again. Bear on hard ; can 't you make a 

 little scratch with your knife-point, or the end of a file? "I 

 believe I do make a little impression on it." Well, then, it is 

 not quartz. Now take another look at it. Compare it with 

 the quartz grain by its side. Is its surface broken and irreg- 

 ular? "No," you say, "it is flat." Hold it then so as to 

 reflect the light from the window. Is the reflection as bright 

 and glassy as the reflection from the quartz ? "I think there 

 is a little difference." You see, too, it is an unbroken re- 

 flection, while that from the quartz is not uniform, in conse- 

 quence of the uneven surface. There is also another point; 

 this mineral appears to be a fragment of a crystal ; you can de- 

 tect one or more edges or angles. It is not so with the quartz. 



