200 EEV. J. F. BLAKE ON THE 



this district, in which the junction of the Purbeck and Portland is 

 said to lie, the one half of the stone being of marine and the other 

 of freshwater origin. I do not know on what evidence this rests : 

 my own observations, possibly too brief for the purpose, did not 

 reveal any such united stratum ; nor do I read any proof of it in 

 Pitton. But it induces me to give an account of the Purbeck beds 

 seen in the great quarry of Chicksgrove overlying the Port- 

 land. 



Section of Rocks above the Portland between Chilmai^Tc and Lealam. 



ft. in. 



a. White finely laminated limestone 3 



b. Black " surface-soil," with many rounded pieces of limestone and 



silicified trunks of trees 1 foot 6 inches to 2 



c. Creamy irregular limestone, in some places turning into a lighter 



" siu*face-soil " above, with larger pieces of limestone, in others 

 having a vacuous limestone at the top 6 



d. Oalc-tuff passing into the above 1 4 



e. Brownish coloured very clean-grained oolite without fossils, and with 



strongflints at the top in some places 8 



f. Irregular botryoidal limestone, passing into a consolidated brash and 



sometimes chalcedonic 4 6 



ff. Brown clay with small renames fossils and shell-fragments 3 



This does not agree exactly with Dr. Fitton's account. The details 

 doubtless vary in different quarries. He alludes to a bed occupy- 

 ing the place of 6 as " a dirt bed," and remarks upon the flints of e ; 

 but he has not, apparently, seen g and the first few feet below it, 

 as he states that " the freshwater strata here rest immediately upon 

 a bed containing marine fossils without the intervention of any clay 

 or dirt." Certainly none of the above strata contains marine fossils, 

 while the bed immediately below g does. Hence I conclude that, 

 whatever the appearances may have been in Dr. Pitton's time, a 

 careful examination will always show that the Purbeck is divided 

 from the Portland by a well-defined line. It would, indeed, be 

 remarkable if it were otherwise, and the events which should make 

 it so almost inconceivable. The Portland series, therefore, com- 

 mences with 



No. 1 (PL yill. fig. 4). This is a fine-grained brown oolite, the 

 grains having no intervening cement. It is a remarkable rock ; and 

 e is probably derived from the denudation of it in the neighbour- 

 hood. The lower part of this is shell-brash, full of empty casts of 

 Cerithium jportlandicum &c. This cannot be the junction-rock 

 alluded to above, as the upper part of it could not well be proved 

 to be of freshwater origin. Thickness 1 foot 8 inches. 



No. 2. A thick mass of rock, finely oolitic at the top, where it 

 contains Cerithium ^ortlandicum, and becoming softer and changing 

 to a fine shell-brash below. There are also vacuous bands here. It 

 makes a magnificent white building-stone, very free and soft when 

 first extracted, and rings under the hammer like a bell. The simi- 

 larity of these two rocks to the Whit bed and Eoach of Portland is 

 almost too striking, as the latter are not in character even at Up way. 



