INTRODUCTION TO THE ROCKS. 37 



mass of smaller grains. See another rock, less coarse, with 

 grains white, pink, and black. See still another with grains 

 all nearly alike. See one rock nearly a uniform white ; 

 another, bluish ; another, reddish ; another, nearly black. See 

 one rock with numerous black shining scales; another, with 

 smoky scales; another, with silvery scales. Now, all these 

 differently colored constituents of the rocks are so many differ- 

 ent minerals. Rocks are composed of minerals. Some rocks 

 have two minerals ; some, three ; some, four ; and some, only 

 one. The particular name of a rock depends on the min- 

 erals in it. As soon as we know the minerals, we can call 

 the name of the rock. Now, sit down and take a lesson in 

 minerals. 



Do you see this white flint rock, composed throughout of 

 one kind of mineral ? That mineral is Quartz. It is the hard- 

 est of all the common minerals. Try to scratch it. You see 

 the point of steel makes no impression on it. Rut it leaves a 

 black mark. The Quartz wears away the steel. When one 

 of these bowlders is thus composed entirely of Quartz, its 

 name is Quartzite. There are many Quartzites, as there are 

 many Smiths and Joneses. Let us learn the other part of the 

 name. Look at these uniformly colored quartzites white and 

 gray. You see one is composed of distinct grains; this is a 

 Granular Quartzite. One has the grains almost completely 

 melted together or confluent ; this is a Vitreous Quartzite. One 

 contains pebbles ; this is a Conglomeritic Quartzite, or simply a 

 Conglomerate. Another has some of its pebbles red ; this is a 

 Jaspery Conglomerate. You will find quartzites exceedingly 

 abundant ; and you will find grains of quartz in many other 

 rocks than quartzites. In fact, quartz is most abundant of 

 all minerals. 



Conglomerates always excite curiosity especially if the 

 pebbles are of different bright colors. Two large masses of 

 conglomerates of different sorts lie on the campus of the Uni- 

 versity of Michigan souvenirs of two graduating classes. 

 One is a jaspery conglomerate from the north shore o Lake 

 Huron. It weighs six tons. The plum pudding, as big as 



