03 



OF THE PRIMARY ROCKS. 



As already stated, the extent of this portion of the state is not 

 accurately known. Its eastern limit coincides in general with a 

 transverse line crossing our principal rivers at their lowest falls. 

 Various species of gneiss rocks and granite are exposed at these 

 points, some of which are beginning to be known for their valuable 

 qualities as a material for building. At Richmond these strata occur 

 in heavy beds inclining at various angles to the east, and are well 

 exposed along the canal for some distance above the city. The 

 rock at Richmond is a grayish-white gneiss of great density, and 

 of such a degree of hardness and toughness combined as to impart 

 the double advantage of durability and facility of being wrought. 

 Its predominant constituents are felspar and quartz. Dark mica 

 is sparsely distributed through it, so that its prevailing colour is a 

 light gray. It is in consequence of its large proportion of felspar 

 that the rock in many places is observed to be decayed to so great 

 a depth below its original surface. Those varieties will be found 

 most durable for architectural purposes in which this excess of fel- 

 spar does not exist. At the falls of the Appomattox near Peters- 

 burg, we find a well characterised variety of granite. It possesses 

 a coarse crystalline structure, and consists of yellowish-white fel- 

 spar in distinct crystals, with a smaller proportion of quartz and a 

 trivial amount of white or light-coloured mica. Other varieties of a 

 more compact and fine texture occur in the same neighbourhood. 

 At some distance farther west, in general the rock is a gneiss, con- 

 taining a much larger proportion of mica. In the upper part of 

 Goochland, and the other counties at corresponding distances above 

 the falls, gneiss of a more compact texture prevails. A quarry of 

 this at Columbia furnishes an admirable building material. Its tex- 

 ture is nearly that of a compact sandstone, and it is coloured of 

 various shades of gray from the scales of black mica which it con- 

 tains. A gneiss of a very peculiar description occurs in Willis' 

 mountain, Buckingham, associated with several interesting minerals. 

 The rock is generally of a rich pink or purple, owing to a large pro- 

 portion of oxide of iron disseminated through it, causing it moreover 

 to be very ponderous. The mica and augite, which appear to be 

 large components of the mass, give it a very brilliant aspect, espe- 

 cially when recently fractured. It is sometimes studded with mi- 



