88 



SECONDARY ROCKS. 



of creatures who should afterward exist on its sur- 

 face, must strike the least inquiring. To this sub- 

 stance England owes, in no small degree, her com- 

 mercial prosperity, and it is destined to enrich our 

 own country beyond all calculation. 



The transition and the lower secondary rocks of- 

 ten present the same external appearance, but they 

 differ in the nature of their fossils ; the former con- 

 taining animal, the latter vegetable remains. An 

 iron ore, called clay ironstone, is generally found 

 in the shale of the coal measures, either in layers 

 or courses of nodules ; these, indeed, are said to be 

 of more regular recurrence than the sandstones 

 and shales, which are more or less variable. 



The medial, or lower secondary order of rocks, is 

 made up of four series : 



1. Millstone Grit and Shale. 



2. The Coal Measures. 



3. Carboniferous Limestone. 



4. Carboniferous Sandstone. 



Millstone grit is composed of angular fragments 

 of quartz and feldspar, held together by a hard ar- 

 gillaceous cement. The shale is distinguished from 

 clay slate by being an aggregate of minute particles, 

 instead of being wholly made up of a single mineral 

 species. These rocks alternate with each other 

 without any regular order ; they may lie either 

 above or below the coal measures. The stratifica- 

 tion of millstone grit is sometimes difficult to detect, 

 on account of the thickness of its beds and its re- 

 semblance to some of the rocks not stratified. It 

 contains but few organic remains, and those of a 

 vegetable kind. It however abounds in minerals, 

 such as galena (lead), blende (zinc), pyrites (sulphu- 

 ret of iron), copper (carbonate), sulphuret of mercury, 

 specular iron, manganese, &c, besides phosphate of 

 lime, fluor spar, and sulphate of baryta. 



The carboniferous limestone is usually compact, 



