186 



the upright position, and apparently in the places where they ori- 

 ginally grew. 



The author found thin beds of clay, with more or less admixture 

 of mechanically divided matter, alternating in several other instances 

 with the fissile limestone at the lower part of the Purbeck forma- 

 tion. Even the " Cap", which in Portland is generally a continuous 

 mass of limestone six to eight feet thick, without stratification, in- 

 cludes, in other places at its lower part, alternations of thin strata of 

 clay. 



The "Cap" is for the greater part compact j but it includes ca- 

 vities lined with botryoidal carbonate of lime, and in other respects 

 resembles very strongly the travertine of Italy. In the clay or dirt 

 beneath it, no trees are found along with the Cycadese, in the Isle of 

 Portland ; but the author observed near Poxwell, on the north-east 

 of Weymouth, part of a silicified trunk, in a bed of " dirt", which he 

 thinks may, not improbably, be inferior to the "Cap", and, conse- 

 quently, the same with the lower of the two beds which in Portland 

 afford the Cycadeee. 



The author ascertained, on attentive examination, that casts of one 

 or more species of Cypris exist throughout the whole series of the 

 slaty limestone beds above the Portland stone : the boundary of the 

 two formations being, as Mr. Webster had supposed, immediately 

 below the "Skull-cap": and, generally, the portion of the Purbeck 

 formation which adjoins that of Portland, may be said to consist of 

 freshwater limestone, alternating with thin beds of clay and mecha- 

 nically worn matter; — two of which beds, at least, contain the re- 

 mains of plants standing in the places where they grew : the whole 

 reposing upon strata which abound remarkably in marine shells. 



The top of the Portland series, in which these shells are so abun- 

 dant, has many points of resemblance to the recent agglomerated 

 limestones of Bermuda, and of the shores of Australia, and other 

 places in low latitudes ; a fact which accords with the supposition that 

 this portion of England was for some time in the condition of a bank, 

 very near the surface, — or of an island of small height above the sea. 



The inferences from the new facts mentioned in this paper, the 

 author remarks, do not invalidate the conclusions of previous ob- 

 servers ; showing only that land must have existed and produced 

 vegetation, above the present site of Portland Island, before the de- 

 position of the upper of the two beds, which contains the trees and 

 Cycadese. The whole of the freshwater strata seems to have been the 

 deposit of a lake, or an estuary, of freshwater, in which (whatever 

 was the cause of the alternations), the waters must have deserted the 

 strata previously accumulated at their bottom, at two successive 

 periods at least ; in each case during a space of time sufficient for 

 the growth of Cycadese ; and in the latter of the two cases, of trees 

 also, — upon the surface of the land thus exposed. 



In conclusion, the author suggests, that the Isle of Portland should 

 be visited from time to time, and frequently, by geologists ; since 

 all the principal appearances of interest are presented by that part 

 of the strata which it is necessary to remove, in order to obtain the . 



