THE YOUNG NATURALIST. 



135 



Professor Hull, in the Quarterly Journal of the Geological Society, Yol. 

 XVIII, has very clearly pointed out the mutual relation of these groups 

 to each other. He shows that the calcareous strata are essentially of organic 

 origin, and the other sedimentary rocks are essentially of mechanical origin, 

 and thai the former may be regarded as occupying neutral ground between 

 the areas of dispersion of the latter series. He proves that life in the sea 

 cannot be so abundant when the water is loaded with sedimentary matter as 

 when it is comparatively clear, and hence he concludes that the sedimentary 

 materials from the disintegration of the land prevents the formation of lime- 

 stone. Now as long as the earth's surface at any particular locality remains 

 quiescent, that is, does not undergo either upheaval or depression, the areas 

 of deposition will remain relatively the same in the three great groups ; but 

 when an upheaval takes place, the arenaceous or argillaceous area will en- 

 croach upon the calcareous area, and when subsidence occurs the reverse is 

 likely to be the case. 



As there are marine formations of all ages, and the most rational idea 

 of a geological period would suppose it to have begun with a change in the 

 land surface, as well as in the fauna and flora, and to have ended with another 

 change. Professor Hull thinks we may regard each complete geological for- 

 mation as consisting of argillaceous and arenaceous deposits in its earlier 

 and later stages, with calcareous deposits near the middle of the period. 

 Bearing this in mind, it may be interesting to notice how many formations 

 would seem to confirm the views of Professor Hull by having this tendency 

 to a threefold division with calcareous deposits in the central position. 



In the Cambrian and pre -Cambrian rocks which have been very much 

 disturbed and metamorphosed, we cannot distinctly mark this arrangement ; 

 but in the Upper Silurian we find this threefold division very clearly defined 

 in the Wenlock and Ludlow formations. 



In the Devonian of Devon and Cornwall we find the middle group consist- 

 ing largely of limestone, while the upper and lower divisions consists mainly 

 of sandstones and slates. 



The Carboniferous system is distinguished by the thickness of its lime- 

 stone, which has its lower limestone shales, and above it the Yoredale shales 

 and sandstones. 



The Permian system in the north of England are developed as follows : — 

 Upper Permian (Sedimentary) 50 to 100 feet. 

 Middle „ (Calcareous) 600 „ 



Lower „ (Sedimentary) 100 to 250 „ 



