GEOLOGY 10H SPRING. 51 



this individual variation has become so great, that we are forced to consider 

 the form, which has become separated from the parent stock, as a sub- 

 species, and this, in turn, in progress of time, becomes a species. 



Thus one species of animal, or plant, has Dec ome derived from another. 



This is evolution, and this undoubted method of the derivation of spec- 

 ies and higher groups, among living organism, is now recognized by nearly, or 

 quite all, naturalists throughout the world. 



GEOLOGY FOR SPUING 



GRANITE, SANDSTONE, AND LIMESTONE CONSIDERED. 



BY 



W». D. Macpiierson. 



The beginner in any science is nearly always bewildered by the vast 

 array of facts or specimens that science presents to him, but a teacher 

 should always aim to show how really simple geology or zoology is when 

 we get at the few fundamental, facts. More definite knowledge later, as 

 the study is pursued, brings us all the array of facts that first startled 

 us. 



Nothing is more confusing for a beginner in the study of geology, 

 than to go through a first class museum and see ten thousand specimens 

 of rocks of all colors and forms. He at once says, " I can never learn all 

 that." 



In the present article, for instruction sake, I intend to take just the 

 other extreme, and tell my readers that there are only three kinds of 

 rocks in the world, and all the great variety we see are modifications of 

 these three. That's simple, surely. 



You have all seen granite, slate, and marble. Well, there are in- 

 numerable kinds of granite, as many modifications of slate, and an endless 

 variety of organic marble and rocks, so that is how the museum gets all 

 its variety and numbers. 



Any boy or girl of ten years, can find quite readily a chip of gran- 

 ite, a pieceof slate, and a piece of marble, and these three rocks will 

 form our museum for the present. Instead of a hurried survey of a hun- 

 dred rocks, we will learn a score of facts about these three rocks. 



We consider granite here in a very broad and general sense. The 

 student will later learn the importance of such rocks as gneiss, diorite, di- 

 abase, etc., which form so much of the earth's crust, in association with 

 granite. / 



Starting from the granite crust of the earth, so to speak, it is quite easj 

 to comprehend how other rocks were built up upon that. The first land 



