Vol. 70.] OF DUEHAil MAGNESIAS LIMESTONES. 239 



Upper Shell-Limestones. 



Hesleden Dene. — The highest part of the reef. 



XXIII. A friable, bedded, very porous, yellow rock, associated 

 with powdery material. Fossils scanty, and somewhat obliterated. 



XXIV. A bedded rock similar to the aboA r e, but the powder 

 largely removed and the rock slightly segregated. It shows much 

 internal deformation. Fossils plentiful in the less compacted parts. 



XXV. Bedded, partly massive, yellow rock, closely associated 

 with the highest Shell-Limestones. It is full of hollow stellate 

 cavities. 'No fossils. 



A stained section shows it to be a fine-grained oolite ; the 

 concentric structures are almost masked, owing to recrystal- 

 lization of the dolomite. The nuclei of the oolites are finely- 

 crystalline dolomite-aggregates dark with impurities. Calcite 

 occurs disseminated through the slide. 



XXIII. XXIV. XXV. 



Insoluble residue 0-31 0-27 0-74 



Fe,0 3 , etc 0-25 039 0-23 



CaC0 3 54-40 59-09 54-71 



MgC6 3 45-55 40-05 45-06 



SO. tr. 0-71 0-06 



Totals 100-51 100-51 100-80 



Calcite 0-05 11-41 1*06 



Dolomite 99-90 87'73 98-71 



XXVI. Castle Eden Dene. — Ivy Rock section. A thinly- 

 bedded, very porous and friable, yellow rock with casts of small 

 fossils. It underlies a typical bryozoonal dolomite. 



XXVII. Black ha 11 Rocks. — A large - grained pea-oolite, 

 overlying the Shell-Limestone conglomerate. 



A section shows large concentric oolites surrounding nuclei of 

 small dolomite-aggregates. The matrix of the oolites shows a 

 good deal of calcite. The concentric material is practically 

 pure dolomite, but contains specks of deeply- staining calcite. 

 (See PI. XXXVII, fig. 4.) 



J< XXVIII. Blackhall Rocks. — Shell-Limestone conglomerate. 

 A rock apparently very largely made up of bryozoa of the genus 

 Thamniscus, the traces of which are almost obliterated. 



xxvi. xxvn. xxvui. 



Insoluble residue 0-21 0-57 0-29 



Fe 2 3 , etc 0-23 0-39 0-17 



CaCO, 55-17 64-66 54-16 



MgC0 3 45-04 34-57 45-75 



SO :! 0-11 tr. tr. 



Totals 100-76 100-19 100-37 



Calcite 1-55 23-50 0-00 



Dolomite 98-66 75*73 99-91 



