278 J. F. BLAKE AND W. H. HTTDLESTON ON 



Section of Lower Corallian Beds in Gillingham Cutting, 



ft. in. 



1. Brown, oolitic, gritty limestone, with Pecten lens and Echino- 



brissus scutatus C> 



2. Blue marl 2 



3. Ferruginous, oolitic, gritty limestone 6 



4. Blue marl, with numerous small oysters 3 



5. Purple rubbly calcareous grit, with fossils. Pecten articulatus ? ... 8 



6. Blue marl, with large spheroidal doggeri, with Ostrea dilatata, 



Serpula trl.-arinata, &c 12 



7. Blue-black calcareous grit in two or more solid blocks, with Mya- 



cites in a natural position and covered by interlacing ramose 

 bodies, Echinobrissus scutatus, &c 2 



8. Dark blue sandy marl, containing at various levels immense sphe- 



roidal doggers of calcareous grit, and sometimes thin layers 

 of alternating sand and clay. Myacites decurtatus, Ostrea soli- 

 taria 18 



The stratigraphy, lithology, and fossils, all prove to us where we 

 are in the series here, and show how very little calcareous grit is 

 to be found among the more argillaceous base of this district. 

 While the top bed is here oolitic, and therefore belonging probably 

 to some portion of the Sturminster section, the lowest bed shows 

 every sign of continuing downwards into the Oxford Clay, in which, 

 indeed, there are hard bands, as seen at the western opening of the 

 tunnel * ; so that we see all that is to be seen. We are not, in this 

 district, too far from the Weymouth area to prevent our comparing 

 these grits with the jNothe Grits, which, it will be remembered, 

 were thinning out and almost disappearing to the north. 



This fundamental divergence from the general type of Corallian 

 beds, viz. the absence of a considerable base of calcareous grit, 

 seems to have affected the subsequent deposits ; for argillaceous con- 

 ditions made themselves largely felt during the formation of the 

 lower oolitic beds. Thus, after the production of the overlying clay, 

 we find in this district, not a new series of grits, as the Bencliff Grits 

 in the Weymouth area, but a set of oolite marls and pisolites, which 

 in places, indeed, become entirely oolitic. Indeed the second 

 remarkable feature of the North-Dorset district is the enormous 

 amount of calcareous matter it contains, which, when impure, is in 

 almost every case mixed with clay and not with sand. This would 

 indicate a remoteness from the source of detritus, and a proximity to 

 some coral reef, or other organic formation, which has been in this 

 case situated to the west. We may remember that in the Wey- 

 mouth area the calcareous beds become thicker in that direction. 



It is by this idea that we must interpret the succeeding marls 

 and pisolites (No. 6). The state of these beds and their rough 

 though small fossils seem to indicate that they are the result 

 of irregular currents bringing material that had been rolled about 

 for some time in a calcareous ooze, the origin probably of the 

 pisolite, and gives us an idea that these North-Dorset beds are 



* The colouring of the Survey Map is hereabouts not quite correct, 



