64 A. J. Jukes-Browne — Ocalk-pebhles on the 



this request he kindly sent me the five slides, as well as two others 

 subsequently made, and a few of the other pebbles. Some of the 

 latter were too much e;iten away by boring animals to allow of 

 cutting, but Mr. W. Hill has succeeded in making slides fiora six 

 of them, and to liis kindness in so doing I owe the opportunity of 

 examining their structure. Mr. Hill has also examined all the slides 

 and concurs in the conclusions at which I have arrived. 



Mr. Worth's Slides. — It will be convenient to deal with these 

 first. M. 14«: This is the specimen which was specially described by 

 Mr. Worth, and it is undoubtedly comparable with the Chalk Rock 

 of the midland and southern counties and with the rocky beds which 

 occur in the zone of Hoi aster planus in Devonshire. No one who was 

 familiar with tbe structure of the different zones of the Chalk could 

 mistake it for Melbourn Hock. It exhibits the same variety of 

 included organic fragments which is so constant a character of the 

 Chalk Rock, and it contains numerous small grains of yellow-green 

 glauconite, a mineral which is of frequent occurrence in the Chalk 

 Rock, but is never present in the Melbourn Rock. 



Again, the material of the included nodule differs from that of the 

 surrounding substance in the same way as the included nodules of 

 the Chalk Rock differ from the mass of that rock. In each case 

 the component materials are the same, but there is a larger proportion 

 of the fine-grained matrix in the nodule than in the surrounding 

 chalk, and rather fewer of the larger fragments, such as pieces of 

 shell and plates of Echinoderms. In other words, the nodules appear 

 to be portions of the same deposit, but sifted by current action and 

 indurated before being embedded in a fresh deposit. 



The included nodule in the dredged pebble contains grains of 

 glauconite, and it is in every respect similar to those of the Chalk 

 Rock and different from those in the Melbourn Rock. 



Mr. Hill and I possess slides of Chalk Rock from Cruxton, near 

 Maiden Newton in Dorset, from Combe Beacon, 3 miles north-west 

 of Chard, from Membury, north of Axminster, and from Pinhay, near 

 Lyme Regis. The first two are typical Chalk Rock with glauconite 

 grains, and are similar in structure to the Channel pebble. The 

 third does not contain glauconite, but is an interesting slide, as it 

 shows areas of typical Chalk Rock material passing gradually into 

 others with a larger proportion of fine matrix and fewer shell 

 fragments, the organic remains in the latter being chiefly 

 foraminiferal cells and 'spheres', with sponge spicules, small 

 Foraminifera, and shell debris (as in the nodules above-mentioned). 

 The rocky chalk at the base of the planus zone at Pinhay shows the 

 usual characters of Chalk Rock, but the glauconite grains are small. 



The other four pebbles cut by Mr. Worth differ from that above 

 described in not containing any glauconite and in having a larger 

 proportion of the fine matrix with its complement of spheres and 

 foraminiferal cells. Three of them resemble one another and the 

 first one (M. 14«) in the variety of their organic contents, and 

 consequently I think they may be referred to the zone of H. planus. 



These slides may be described as showing a fine chalky matrix 

 through which are scattered many cells and spheres, with a large 



