118 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 21, 



passage from the Portland into the Purbecks ; that " the top beds of 

 the Portland stone are purely marine, and the lowermost beds of the 

 Purbeck are purely freshwater, containing Cyprides, Valvata, &c.'' 

 The true nature of No. 2 consequently involved the questions of the 

 gradual or abrupt succession of the Purbeck group — whether the 

 dirt-bed (No. 3) reposed directly upon the Portland Oolite, or 

 whether here, as elsewhere, there was a freshwater bed intercalated 

 between the lowest dirt-bed and the Portland. This case could alone 

 be decided upon by palseontological evidence, and Prof. E. Forbes most 

 kindly undertook to examine the fossils. " After a close examination 

 of the specimens sent (writes Prof. F.), my impression is that they 

 are of freshwater origin, and belong to the lowest beds of the Pur- 

 beck series, and contain Valvata and Cyrena.'^ 



I have consequently marked No. 2 as Purbeck ; the apparent dis- 

 crepancy between the mineral and fossil evidence thus adduced may 

 I think be easily reconciled. 



It will be seen that the beds (4 and 6) were either originally formed, 

 or (subsequent to their deposition) structurally modified, hj chemical 

 forces ; No. 2 has also a compact and subcrystalline texture indicating 

 similar chemical influences. Now it is very easy to conceive that after 

 the elevation of the Portland oolite above the sea, the Purbecks may have 

 been forming upon the yet unindurated surface of the Portland, and 

 that after the deposition of the beds exhibited in the section the per- 

 meating gases, absorbed by water, could not only impart to No. 2 its 

 subcrystalline character, but convert and cement the lowest Purbeck 

 and the uppermost Portland (both calcareous rocks) into one homo- 

 geneous body The induration of aqueous strata, depending as it does, 

 not merely upon the absorption of carbonic acid gas, but (among other 

 things) upon the loss of watery particles, could not have proceeded 

 while the upper oolites remained submerged, and, as we know not 

 what period elapsed between the formation and elevation of the Port- 

 land, the view I have suggested is not at all incompatible with that 

 evolution of time which must have occurred between the Oolitic and 

 Wealden seras. 



3 and 5. These are true dirt-beds, and fossilized trees have been 

 found in them. 



4 and 6. These Purbeck beds appear to me to have been originally 

 formed by deposition from water charged with carbonic acid gas. 

 They are decidedly tufaceous. 



As I did not procure positive evidence of either the fresh or brack- 

 ish water character of these beds, I have marked them accordingly. 

 Most probably, however, they will prove to be freshwater. 



6. This bed has carbonaceous alternations at the upper part, show- 

 ing on a minute scale the oscillations of the then existing surface 

 during shorter intervals of time. 



7. This remarkable bed consists of beach-pebbles (with a few 

 chalk-flints), rounded by continued sea-action. The quarryman said 

 it extended about \ mile North of the Bill. 



It is to be remarked that the surface of the island, when carefully 

 examined, does not dip regularly down towards the South, like an 



