134 



THE YOUNG NATURALIST. 



sent by those who have kept careful diaries, are worth incalculably more 

 than those who depend upon memory, or casual notes for their information, 

 inasmuch as accurate dates and localities and tolerably, accurate numbers are 

 absolutely essential. Let no one be afraid of writing too much at length 

 about any species. What is wanted is a mass of accurate and somewhat 

 minute information, from which to deduce conclusions, and the more of this 

 information there is forthcoming the better. 

 Bockleton Court, Tmbuvy. 



GEOLOGICAL NOTES. 



By JOHN DALE, F.R. Met. S. 



Of all the Geological formations, from the Laureutian to the Post Tertiary, 

 sedimentary constitute the greater part ; if we include the metamorphic 

 rocks, which were, no doubt, sedimentary, we shall find that fully nine-tenths 

 of all the rocks which are exposed on the earth's surface may be called sedi- 

 mentary. The mineral character, structure, and origin of these rocks have 

 not received that attention and investigation which their importance and 

 magnitude demand, many of our text books being lamentably deficient in 

 clear and intelligent descriptions of them. 



In Ly ell's " Student's Elements of Geology " we find five chapters, extend- 

 ing over eighty pages, devoted to a description of the mineral character, 

 structure, and origin of igneous rocks, while the same features in the sedimen- 

 tary rocks, apart from the fossils they contain, do not occupy more than one- 

 third of that space. In Rutley's " Study of the Eocks " we find about the 

 same proportion of space ; in the second part of the book on Descriptive 

 Petrology, igneous rocks take up one hundred pages, while sedimentary rocks 

 are dismissed in thirty pages. More attention, however, has been given to 

 the latter class of rocks in Geikie's "Text Book of Geology/' and any attempt 

 to extend our present limited knowledge of the character and origin of sedi- 

 mentary rocks, we believe to be worthy of commendation. 



Any tyro in Geology knows that the bulk of the sedimentary rocks may be 

 divided, according to their mineral character, into three groups, viz. : the 

 argillaceous, the calcareous, and the arenaceous — or clays, limestones, and 

 sandstones ; but the relation of these groups one to another is not so clearly 

 seen or understood, and it is with the intention of elucidating, this that 

 the present article has been written. 



