102 THE WONDERS OF GEOLOGY. Lect. I. 



sand, shells, and corals, cemented together by calcareous 

 spar ; and others by ferruginous infiltrations : but this 

 specimen is a congeries of rounded flint pebbles, imbedded 

 in a siliceous paste. This conglomerate is commonly 

 called Hertfordshire pudding-stone, and was formerly in 

 much request with lapidaries ; for the cement being as 

 hard and solid as the pebbles themselves, the stone admits 

 of being cut and polished into a great variety of ornaments. 

 The formation of this rock must have resulted from a 

 stream of siliceous matter having flowed through a bed of 

 gravel, and cemented the pebbles into a solid mass, while 

 those portions which the liquid flint did not reach, remained 

 as loose water-worn materials. 



But there are many very hard sandstones composed 

 entirely of siliceous granules, which appear to have been 

 formed simply by the effect of great pressure, there being 

 no cementing medium ; the rock, when broken, appearing 

 like the fractured surfaces of fine sugar.* 



It is not my intention in this lecture to dwell on the 

 silicificationf of the remains of animals and plants ; it will 



* A consolidation of this kind can be produced artificially. In the 

 experiments made for the trial of the strength of gunpowder, leathern 

 bags filled with sand are put into the mortar which is to receive the 

 cannon ball, propelled by the powder from another gun at the dis- 

 tance of only fifty feet. The sand is frequently compressed by the 

 percussion of the ball, into a mass of sandstone, sufficiently firm to 

 remain solid and bear handling ; and this sandstone is perfectly free 

 from any cement. The consolidating power of great pressure has 

 lately been ingeniously applied to various purposes ; among others, 

 to the formation of tesserae from porcelain earth, and of graphite or 

 plumbago, fit for the finest pencils, from the rubble of rejected ore 

 formerly thrown by as useless. This rubble is reduced to an impal- 

 pable powder, and then subjected to great pressure in moulds, and the 

 most pure graphite is the result : this prepared mineral is now gene- 

 rally employee! for the best lead pencils. 



f Petrifaction by flint. 



