ZONE OV LYTOCERAS JURENSE. 139 



berry Bank, Ilminster, already described at p. 119 ; where the rock consists of alternate 

 strata of clay and stone, in which are found Harpoceras insigne, Harpoceras variabiles 

 and H. opalinum, var. Moorei. These Ammonites have not migrated, as some geologists 

 state, from the Upper Lias sea into the Cephalopoda-bed of the sands above, inasmuch 

 as neither of the species are ever found in the Seryentijium and Bifrons zones, but are 

 truly characteristic forms of the Jurense-zou^i, of which they form the leading species. 



South of the Mendips the sands attain a thickness of 160 feet, whilst north of that 

 range they diminish much, are extremely variable, and are absent in several localities. 

 Near Bath they are about 40 feet thick, and are well exposed in the railway tunnel under 

 Combe Down ; also at Midford, near the late Dr. William Smith's house, hence the 

 origin of the name, " Midford Sands." 



In Dorsetshire this zone is found in several localities. I have carefully examined 

 it in the following ; at Chideock Hill, between Bridport and Charmouth, it is exposed 

 near the summit, and consists of a very fine sandy rock of a light yellow colour, some- 

 times micaceous, in which I found good specimens of the following Ammonites, 

 Harpoceras insigne, Harp, variabile, and Harp, opalinum, all characteristic species in 

 good preservation. From Burton Bradstock I have obtained some large Harp, opalinum 

 and Harp, variabile in a fine yellow sandy matrix ; the sands here attain a considerable 

 thickness, and have several inconstant bands of sandstone intersecting the deposit. 

 At the east side of Bridport Harbour there is a magnificent coast section of the sands, 

 estimated at upwards of 200 feet in thickness ; and between Bridport Harbour and 

 Burton Bradstock there are several good quarry sections, which show the upper rag- 

 stones of the Inferior Oolite resting upon the Cephalopoda-bed. These two rocks so 

 closely resemble each other in their petrology, that, but for the organic remains they 

 contain, it would be impossible to separate them. Fortunately, however, the Inferior 

 Oolite contains many Ammonites, Bivalve shells, Echinides, and Corals, which are 

 typical of the Lower Oolitic beds, so that the divisional line between them can be surely 

 drawn by palseontological evidence where lithology fails to do so. At the west side a 

 fault has depressed the sands, which are here overlain by thin beds of Inferior Oolite and 

 Fuller's Earth. Near Yeovil several instructive sections of the sands are exposed in the 

 railway-cuttings around that town, and in the neighbourhood of Sherborne the sands are 

 about 140 feet thick. 



My friend Dr. Lycett, in his notes on the Ammonites of the sands intermediate to the 

 Upper Lias and Inferior Oolite, intending to afford a concise analytic examination of their 

 natural-history, characters, and geological distribution, assumed that it had been generally 

 considered that all the species belonged to the lower horizons of the Upper Lias, but 

 this was not the teaching of my Memoir, in which, by many sections and carefully 

 prepared lists of the palaeontology of the Upper-Lias sands^ in the counties of 



' " Notes on the Ammonites of the Sands intermediate to the Upper Lias and Inferior Oolite," 

 'Trans. Cotteswoid Naturalists' Club,' p. 3, 18GJ. 



