Transported masses in Boulder-clay 41 



gay, and Impingtoii ; and in the district of Old North Road, 

 Bourn, Comberton, Barrington, and Grantchester, which has 

 been explored by the Sedgwick Field Club, it is quite common. 

 Rhomb-porphyry proper is only known from the last mentioned 

 district but is also recorded outside Cambridgeshire in the 

 gravels of Leighton Buzzard and at Hunstanton, so will 

 probably soon be found elsewhere. 



Transported cakes of soft rocks do not seem to be par- 

 ticularly common among the Boulder-clay of Cambridge, but 

 the earliest described example was that of the Roslyn pits at 

 Ely. There a mass of undisturbed Gault, Cambridge Green- 

 sand and Chalk several hundred yards long occupies an old 

 valley 'ploughed out' of the Kimeridge Clay. At one time 

 Boulder-clay could be seen on all sides of the mass dipping 

 distinctly under it, but now the Lower and Middle Pits are 

 entirely overgrown, and though Gault, Greensand and Chalk 

 can still be observed on the south and east sides of* the Great 

 Pit, the relation between the various members is by no means 

 clear. 



An even larger cake of Upper Chalk occurs among the 

 Boulder-clay of Catworth, Huntingdonshire, and a well sunk 

 to the Lower Greensand, near Biggleswade, Bedfordshire, has 

 passed through another of Ampthill or Kimeridge Clay. The 

 well section has been recently described by Mr Home, who states 

 that the clay is practically undisturbed and is about sixty- 

 seven feet thick. The included septarian beds dip steeply and 

 the mass is both underlain and overlain by Boulder-clay. 



The mass is also interesting as a boulder, for by its fauna 

 and general lithology and by the mode of preservation of its 

 fossils it closely resembles the Lower Kimeridgian beds of 

 Market Rasen (Lincolnshire) and is quite unlike any of the 

 clays of our district. 



The exposures of Boulder-clay are generally of the most 

 unsatisfactory description, and in collecting erratics the arable 

 land of the outcrop generally gives a much better yield than 

 Boulder-clay in situ. 



