Picrite and other Associated Rocks, 417 



existed for calculating the thicknesses of the several parts of the 

 Purbeck series in the Vale of Wardour, and comparing the different 

 subdivisions as developed in that Yale with those exposed in other 

 localities. Putting together 22-24 feet of basement-beds of the 

 Lower Purbeck strata seen in the Wockley section, 21 feet in "Ridge 

 Quarry, and 17 feet in Teffont Quarry, more than 60 feet of Lower 

 Purbeck Beds are actually seen, and allowing for the gap between the 

 Wockley and Ridge sections, 70 feet seems a fair estimate of the 

 average thickness of the Lower Purbeck strata. 



In the Teffont and Chicksgrove quarries, a little over 12 feet may 

 be measured between the basal clay of the Middle Purbeck Beds and 

 the Cinder Bed ; while the great cutting on the Teffont line shows 

 19 feet of strata above the Cinder Bed, and the beds of Dinton 

 cutting may be all on a higher horizon ; so that an estimated 

 thickness of 32 feet for the Middle Purbeck Beds is probably below 

 the mark. 



The clay and sand in the Dinton cutting must be 18-20 feet 

 thick, and between its summit and the top of the second seam of 

 calcareous grit there is a thickness of at least 8 feet. This grit 

 forms the floor of Dinton Well, about 40 feet deep ; hence there is 

 a thickness of at least 66 feet of Upper Purbeck strata, and probably 

 more than 2 and less than 12 feet in addition. 



A comparison is instituted between the Purbeck Beds of the Yale 

 of Wardour and those of the Dorset coast, etc., and some remarks 

 are made upon the physical conditions under which the beds were 

 deposited. 



2. ' On a Picrite and other Associated Rocks at Barnton, near 

 Edinburgh.' By Horace W\ Monckton, Esq., E.L.S., E.G.S. 



The object of this paper is to describe a cutting on a new railway 

 in Barnton Park, where there is an excellent exposure of picrite. 

 It consists of serpentinized olivine, augite, mica, iron oxide, and a 

 little plagioclase-felspar, with a variable amount of interstitial 

 matter. In many respects it comes very near to the picrite of 

 Inchcolm, which island is 4| miles north of Barnton cutting. It 

 differs from the picrite of Bathgate, and the probability is that the 

 Barnton rock is an offshoot from the same magma as that which 

 supplied the Inchcolm rock. 



Besides the picrite other igneous rocks from the same cutting are 

 described — in particular, a rock with porphyritic crystals of a green 

 mineral replacing olivine, or more probably augite, and a great 

 quantity of brown mica in small flakes and crystals. It is suggested 

 that the name of mica-porphyrite might be given to this rock. 



2. * On a Yariety of Ammonites (StepJianocey^as) subarmatus, 

 Young, from the Upper Lias of Whitby.' By Horace W. Monckton, 

 Esq., E.L.S., F.G.S. 



