MICROSCOPIC OBJECTS, 95 



be two specimens of each, one with a fractured surface, 

 the other smoothed and poHshed. • 



Stratified rocks are produced from the abraded parti- 

 cles of primitive rocks, the fragments of shells, coals, 

 indurated clay, &c., deposited in sedimentary beds at the 

 bottom of seas and lakes. They may be formed into five 

 principal groups, viz.: — slates, sandstones, magnesian 

 limestones, oolites and Hmestones. The sandstones em- 

 ployed for the erection of buildings contain about 95 per 

 cent, of sihca; the remainder being variable foreign 

 matter, as oxide of iron, alumina, lime, &c. On minute 

 examination, the quartz is observed in the form of grains 

 cemented together by siliceous matter deposited by water 

 that has percolated through them. They are usually la- 

 minated, and where mica enters into these compositions, 

 the minute plates of that substance are disposed in layers. 

 Tlie cohesion and consequent durabihty of these stones 

 depend upon the quality of the cementing substances, 

 the grains of sihca being themselves almost indestruc- 

 tible. The magnesian Hmestones, which are composed of 

 carbonate of magnesia and carbonate of Hme, when 

 united in the proportion of their chemical equivalents, 

 are valuable for building purposes. The principal stone 

 used in the erection of the new Houses of Parliament 

 is a stone of this nature from Austen. 



The oolites or roestones are composed of about 92 per 

 cent, of carbonate of lime. They consist of small sphe- 

 rical bodies united together by a calcareous cement. 

 The size of the grains varies considerably. The largest, 



K 2 



