96 THE PRIMARY PERIODS. 



which they are traversed. The vast bulk of their strata 

 "being gneisses and mica-schists, are little fitted for architec- 

 ture ; but their limestones, marbles, and serpentines are often 

 of great beauty and much sought after for ornamental pur- 

 poses ; while their fine-grained cleavable clay-slates afford 

 abundant material for roofing and other multifarious appli- 

 ances. Indeed, it is chiefly in these old metamorphic rocks 

 that serpentines/ variegated marbles, and roofing - slates 

 occur, the mineral transformations to which they have been 

 subjected giving to the former those varied shades of col- 

 ouring and figure, and to the latter that fissility or cleavage, 

 for which they are prized. But if the rocks of these systems 

 are of comparatively little value, their metalliferous veins 

 are rich and numerous gold, silver, tin, copper, antimony, 

 manganese, iron, and other metals being abundant in most 

 primary districts, either in the veins themselves or in the 

 debris that has been worn and washed from them during 

 the course of ages. These primary rocks constitute, indeed, 

 the bulk of all our older hill-ranges, and it is only in them 

 that the slow processes of chemical deposition have yet ela- 

 borated on a grand scale the metallic ores, and the vein- 

 stuffs with which these ores are usually associated. It is 

 true that veins and ores occur in some of the younger for- 

 mations, but not in the same variety, nor with the same 

 richness and persistence, as those that belong to the primary 

 and more highly metamorphic strata. Hence, it may be 

 remarked, the importance of geological investigation in 

 colonial and newly-discovered countries ; the determination 

 of their formations being tantamount to a declaration of 

 their mineral and metallic wealth, or, in other words, their 

 natural fitness for mechanical and commercial development. 



Such are the great primary periods of world-history 

 the Laurentian, the Cambrian, and Silurian. Geologists 



