96 Essay on Geology. 



feet above the level of the sea in height, and is deeply intersected by 

 valleys of excavation. All the phsenomena of this kind, mentioned in our 

 introductory outline, p. 15, may be well studied in the valleys around 

 Bath. In Dundry Hill itself, we have a good example of what is meant by 

 an insulated outlier, (see the wood-cut in the place referred to) j it presents 

 us with the lower beds of the oolite formation, which are of a coarser 

 texturC;, spotted with oxide of iron, and separated from the lias by beds of 

 calcareous sand : the quarries of Dundry will afford us a rich harvest of 

 the organic remains of this formation. The oolite is subdivided into many 

 subordinate beds, but it would only give rise to confusion to specify them 

 all in this place j we need only at present attend to one of the princii)al of 

 them. The Bath oolite and its upper bed, called the cornbrash, sinks be- 

 neath a thick clay, not unlike much of the lias formation along the line 

 from Melksham to Chippenham. In some stony beds associated with this 

 clay, at Kelloways, abundance of some peculiar species of ammonites are 

 found. Further west, rises the ridge of middle oolites, and ranges by 

 Steeple Ashton, Bow Wood, and Calne ; this is also called, from the 

 abundance of madepores which it contains, the coral rag, and offers the 

 exact antitype of one of the coral reefs of the Pacific, so fully described by 

 Mr. Stuchbury in our last number j as he informs us that corals do not 

 extend to a depth of many fathoms beneath the surface, this would afford 

 us a datum for estimating the depth of the ocean at the period when the 

 coral rag was formed. This subject seems to us to be pregnant with the 

 most important inferences, and has not^ we believe, been hitherto attended 

 to by geologists ; since many of the later marine deposits, the chalk for 

 instance, tower high above the coral rag. If the latter must indeed have 

 been at the time of its formation so nearly superficial, the crust of the 

 earth must have subsequently been greatly depressed before the chalk 

 could have been deposited ; but as the general movement was assuredly 

 that of successive elevations of the said crust, our argument would seem to 

 imply very material oscillations. 



The still more recent deposit of the green sand, as it is called, (from 

 the specks of chloritic earth intermingled with it) sweeps over the coral 

 rag south of Steeple Ashton, and advances by Warminster to the long 

 ridge above Stourhead, crowned by Alfred's tower, a very conspicuous 

 object in our panorama from Dundry. The organic remains which may be 

 found about Warminster, are abundant, interesting, and peculiarly charac- 

 teristic of this formation, especially the numerous varieties of fossil alcyonia 

 and sponges. The western extremity of the chalk downs of Wiltshire, 

 may be seen towering above this ridge of green sand, in the hills above 

 Maiden Bradley and Mere, marked objects in our view from Dundry, and 

 within twenty miles. At these pohits we may obtain all the chalk fossils; 

 so that we have surveyed every formation, from grauwacke to chalk, in a 

 panorama of less than thirty miles radius from Dundry as a central point ; 



