Sept. 7, 1888.] 



SCIENTIFIC NEWS. 



221 



and*lithological differences. These divisions are further 

 distinguished by zones of life — that is to say, that at 

 certain periods during the progress of this formation 

 •certain creatures lived more abundantly than at others, 



ing the rocks. The Upper Lias beds pass up into 

 yellow sandy strata, which are typically represented at 

 Midford, near Bath, and were therefore at first named 

 the Midford Sands. Above the sands we get a very in 



Fig. 5.— The Pea Grit as Exposed at Cleeve Hill, near Cheltenham. 



or lived only during the time that certain strata were 

 forming, and then disappeared. Many of these forms of 

 life remain as fossils, and hence it is that we are enabled 

 to make out certain horizons or zones which characterise 

 the Lias. In marking these zones the Ammonites are the 



Fig. 6. — Micro-Photograph of a Section of the Pea 

 Grit, from Cleeve Hill, near Cheltenham. (Enlarged.) 



chief indicators. Thus the Lower Lias is marked by 

 seven Ammonite zones, the Middle Lias by five, and the 

 Upper by three. 



Before referring further to the life of the Jurassic 

 period it may be well to say something more respect- 



teresting bed, called by Dr. Wright the Cephalopoda bed, 

 because of the number of Ammonites and Belemnites 

 which occur in it ; and it is here that some geologists 

 draw the line between the lias and succeeding oolitic 

 formations. 



Fig. 7. — Micro -Photograph of a Section of Bath 

 Stone (Great Oolite.) ( x 22 diams.) 



The term Oolite is from the Greek oon, an egg, and 

 lithos, a stone. The name is suggested from the struc- 

 ture of a large portion of the rock, which is made up of 

 minute granules resembling somewhat the roe of an egg. 

 The Oolitic formation is divided as follows : — 



