226 



THE POPULAE EDUCATOR 



stone of the Permian system ; and then come plains of 

 new red sandstone again, crowned by the escarpment 

 of the narrow strip of oolite, and by that of the cretaceous 

 rocks. 



This structure explains the course of the larger rivers. 

 The principal watershed of the country is the tract of 

 high ground extending from the north of Scotland far 

 into England ; it is nearer to the west coast than to the 

 east, and therefore a much larger area of country is 

 drained towards the east than towards the west. All the 

 larger- rivers with the exception of the Severn and its 

 tributaries run into the German Ocean. The plains 

 which occupy much of the middle and east of England 

 are traversed by many tidal rivers : and from the nature 

 of the country, the construction of canals has been a 

 comparatively easy task. 



II. Geological Distribution of Mineral Products. 



The modes of occurrence of minerals are in veins or 

 lodes, in regular or irregular beds, and in connection 

 with detrital matters. The consideration of the dis- 

 tribution of minerals will be treated of under these 

 heads. 



1. Minerals in Veins. 



Though it is difficult, in the present state of our .know- 

 ledge, to ascertain the .laws regulating the depdsit of 

 metalliferous matter, yet we are able, from general ob- 

 servation of the geological structure of the earth, to say 

 that here search may be made for minerals with some 

 hope of success, and that there exploration will be futile. 

 Hundreds of thousands of pounds have been expended 

 in the opening up of mineral districts ; a large propor- 

 tion of the sum utterly without profit. 



Great Britain possesses a rich supply of minerals ; we 

 have gold, silver, copper, lead, tin, zinc, antimony, nickel, 

 cobalt, bismuth, uranium, chromium, and .other of the 

 rare metals, besides vast stores of iron ; our coal beds 

 are enormous, and earthy minerals are in great variety 

 and value. Statistics show that in Great Britain 350,000 

 persons are actually engaged in mining operations, ex- 

 clusive of quarries of all kinds, and that the produce is 

 of the minimum annual value of 40,000,000. 



The lodes from which we derive our chief supply of 

 metals are almost wholly confined to palaeozoic rocks. 

 Their occurrence may be sketched as follows : 



The Silurian formation in North Wales, in the Isle of 

 Man, in Cumberland, in the lead hills of the south of 

 Scotland, in parts of the Highlands, and in parts 

 . of Ireland, contains metalliferous veins which yield 

 gold, ores of copper, lead, silver, antimony, arsenic, and 

 zinc. 



The rocks of the Devonian formation in Devon and 

 Cornwall contain rich tin, copper, and lead lodes. 



The carboniferous limestone in Derbyshire, ranging up 

 to the north of England through Cumberland and the 

 adjacent counties, also of the Mendips, and in Devon, 

 is the chief depository of our lead ores. The same for- 

 mation contains large and rich deposits of haematite, 

 an ore of iron, as in the Forest of Dean and Somerset- 

 shire. 



Throughout the world, all the metalliferous lodes, 

 with some peculiar exceptions hereafter to be mentioned, 

 occur in stratified or the associated igneous rocks, not 

 newer than the Permian. It is thus that one generalisa- 

 tion in the inquiry is arrived at, viz., that of the period 

 during which the lodes carrying our richer metals 

 were filled. Geology, like the more exact sciences, is 

 capable of advancing philosophical inductions to very 

 important results. Sir Roderick Murchison was en- 

 abled in 1844, from the study of the gold-bearing tracts 

 in Eussia, to predict the discovery of gold in Australia. 

 "Having," writes Sir E. Murchison, "in the year 

 1844, recently returned from the auriferous Ural Moun- 

 tains, I had the advantage of examining the numerous 



specimens collected by Count Strzelecki, along the 

 eastern chain of Australia. Seeing the great similarity 

 of the rocks of those two distant countries, I could 

 have little difficulty in drawing a parallel between 

 them ; in doing which, I was naturally struck by the 

 circumstance, that no gold 'had yet been found' in, the 

 Australian ridge, which I termed in anticipation the 

 ' Cordillera.' Impressed with the conviction that gold 

 would, sooner or later, be found in the great British 

 colony, I learned in 1846 that a specimen of the ore had 

 been discovered. I thereupon encouraged the unem- 

 ployed miners of Cornwall to emigrate and dig for 

 gold, as they dug for tin in the gravel of their own 

 district. These notices were, as far as I know, the 

 first published documents relating to Australian gold." 

 (" Siluria.") 



Influence of Igneous Rocks in the Development of 

 Minerals in Veins. 



Mineral veins occur in igneous rocks as well as in 

 aqueous rocks ; but the intrusion of an igneous mass 

 among stratified deposits appears to have rendered their 

 lodes richer than when conditions otherwise similar 

 obtain. 



Gold is usually found in a quartz matrix, traversing 

 paleozoic shales, chiefly those of the lower Silurian 

 epoch ; and the auriferous lodes are frequently richest in 

 the vicinity of eruptive rocks. But the precious metal 

 is found also in secondary rocks, such as those of Cali- 

 fornia, Peru, etc., yet under circumstances exceptional to 

 the usual mode of association of gold. It appears that 

 where certain igneous eruptions, diorite especially, have 

 penetrated the secondary strata, the latter have been 

 rendered auriferous for a limited distance only beyond 

 the junction of the two rocks ; and it is concluded that 

 all secondary and tertiary deposits (except the auriferous 

 detritus of the latter), not so specially affected, never 

 contain gold. 



The lodes carrying copper and tin in Cornwall and 

 Devon are richest about the junction of the Icillas (local 

 name for the slaty rocks of the Devonian formation in 

 this district), and the bosses of granite, and where they 

 are intersected by granitic dykes, termed elvans. It is 

 worthy of remark that these metalliferous veins have 

 a course or strike nearly east and west, and that these 

 phenomena are not confined to this area, but are exhibited 

 in Saxony and elsewhere. 



Other examples might be adduced, but these will 

 suffice to show that intrusive rocks influence the metal' 

 liferous richness of veins. 



2. Bedded Mineral Deposits. 



These include coal and iron ore of primary importance ; 

 and salt, gypsum, cement-stones, coprolites, iron-pyrites, 

 bituminous shales, etc., of secondary value. 



(a.) Coal occurs in many formations ; it has, been mined 

 for upwards of a hundred years at Brora, in Sutherland- 

 shire, in rocks of the oolitic epoch, and is worked at 

 Bovey Tracey, Devonshire, in Miocene beds. An anthra- 

 cite occurs in the Devonian rocks in Spain ; there are 

 good workable coals of the age of the trias in Virginia 

 and Hindostan, and of that of the lias in Hungary ; and 

 less valuable coals, chiefly brown coals, occur in tertiary 

 strata in Austria and other parts of Germany. But by 

 far the richest and largest supplies are drawn from the 

 carboniferous system in Great Britain, Belgium, United 

 States, Nova Scotia, Australia, etc. 



In Great Britain no coal is found below carboniferous 

 strata, but it does occur -in newer strata. In the mid- 

 land and south-western counties of England, and in South 

 Wales, it is confined to the true coal measures underlain 

 by the millstone grit, locally called the " farewell rock," 

 because, in the language of the miner, when that rock is 

 reached, one bids farewell to the coal. But in the north 

 of England and in Scotland workable coal seams occur 



