igS The Ottawa Naturalist. [February 



every piece of granite ? Which mineral gives granite its prevail- 

 ing color ? When a rock is studied, I should give talks on its 

 uses. The child will look at the next granite tombstone or wall, 

 to see if it is the kind he had in school. He will endeavor to 

 learn something of the methods of quarrying, cutting and polish- 

 ing— in fact will take an interest in one phase of industrial life 

 that he had not thought of before. 



The use of granite as a building stone suggests marble and 

 sandstone, which are used for similar purposes. Let us study 

 their properties, and then their origin. Is marble harder or softer 

 than granite ? Would it be more easily cut ? How is it aflfected 

 by dilute acids? There are certain acids in small quantities 

 always present in the air. Decaying vegetable matter gives off 

 humus acids, which, in presence ot moisture, attack marble. This 

 may be seen in old tombstones overgrown with vines, where the 

 lettering is partly or wholly obliterated. Is marble, then, so dur- 

 able as granite ? In manufacturing cities, more acid is thrown 

 into the air than is normally present. Would it be advisable to 

 use marble for building purposes in such cities ? Would granite 

 be better? Would sandstone have any advantages over either ? 

 Is it easily affected by acids ? Is it cheaper or more easily 

 worked than granite? Finegrained sandstone is better than 

 coarse-grained, for it will not absorb so much water, which would 

 cause chipping in the winter. Iron pyrites is injurious to sand- 

 stone, for, by weathering, it stains the stone and leaves it porous. 



Now, for the origin of these stones, we shall return to granite 

 again, and, in learning these two, we shall incidentally learn a 

 few others. Granite, on weathering, breaks up into fragments of 

 quartz and feldspar. The former grinds to sand, and the latter 

 to clay. Since the clay is more finely divided, water will separate 

 it from sand. These are washed into the sea in different layers ; 

 finally, by pressure and cementing material, the sand becomes 

 compressed into sandstone and the clay into shale. Through 

 further influence of heat and pressure, the sandstone becomes 

 quartzite, and the shale, slate. These few facts can be elaborated 

 by the up-to-date teacher, who will probably know where beds of 

 some of these rocks are visible. Their dip will be explained in 

 teaching the immediate origin of granite and other eruptive rocks- 



