68 ME. B. THOMPSON ON THE GEOLOGY OF THE [Pob. 1 899, 



It now became evident that the blue Boulder Clay was a com- 

 paratively thin intermediate layer between the brown Boulder Clay 

 — where the latter Avas present — and true Lower Lias, being, in fact, 

 little more than the disturbed upper portion of this latter. It was of 

 almost exactly the same colour, and contained about the same fossils, 

 equally abundant, and on the whole in an equally good state of 

 preservation, though some of the larger specimens were smoothed 

 and striated, as were also the larger included limestone-blocks. In 

 many cases it was impossible to determine with certainty whether 

 a fossil came from the blue Boulder Clay or from the Lower Lias ; 

 but, for the reasons stated above, I think that no serious error occurs 

 in my list on account of this ambiguity. 



As before observed, I consider that the undisturbed Lower Lias 

 commences a little south of H (fig. 1, p. 66), at the very bottom 

 of the excavation, and from this point southward for a short 

 distance Ammonites Tumeric or other ammonites of the same group, 

 are fairly abundant. A. semicostatus and A. Sauzeanus come in at 

 about the same place, but continue to occur abundantly for a much 

 greater distance southward. This seems to indicate that A. Turneri 

 characterizes only a portion of the A. semicostatus-zone, a conclusion 

 at which I had previously arrived from evidence obtained elsewhere. 



The phosphatic nodules, already mentioned as coming from the 

 excavation for the foundations of the bridge at I, were afterwards 

 found between I and J (fig. 1, p. 66), as a thin, irregular, but fairly 

 continuous bed at an average height of 4 feet from the bottom of the 

 cutting, and close under the blue Boulder Clay for some distance. 

 This layer of nodules seemed to rise towards the south — that is, in a 

 direction opposite to the dip of the beds ; but the seeming rise might 

 be due to the fall of the cutting in a southerly direction, or to the 

 irregularity of the bed. In any case, I think that this layer may be 

 taken as the boundary between the semicostatus and ohtusus-zones. 



The semicostatus-zone appears to finish at about J (53 miles 

 48 chains), and simultaneously the blue Boulder Clay ceases to yield 

 fossils, though towards and near this point both the blue and the 

 brown Boulder Clays yield Ammonites semicostatus, etc. 



Among the fossils from this zone possibly quite half of the very 

 common species came from the Boulder Clay, though comparatively 

 foAv of these show any signs of the fact, the ribbing, in the case of 

 the ammonites, being extremely sharp. 



The following, among other characteristic fossils, were obtained : — 

 Ammonites semicostatus, Y. & B. ; A. Sauzeanus, d'Orb.; A. Turneri, 

 Sow. ; A. BrooTci, Sow. ; and A. Bonnardii, d'Orb. 



{b) Zone of Ammonites obtusus (figs. 1-2^ pp. Q^ & 70). 



Extending from J to a little beyond M (53 miles 48 chains to 

 54 miles 10 chains), the lower part of the cutting exposed a mass of 

 clay or shale which was especially unfossiliferous. An examination 

 of an exposure 200 to 300 yards long yielded practically nothing 

 that could be identified. As it was obvious that this mass of clay 



