Vol. 51.] SECTIONS IN THE MALVEKN HILLS. 5 



So far, then, as I dare have an opinion, I am inclined to think that 

 we have here a group of bedded acid lavas and tuffs, crossed by- 

 three bands of dolerite. 



The question arises whether these dolerites are flows or intrusive 

 sheets. What little balance of evidence there is seems to me to be 

 in favour of their being intrusive. Though they are fairly well 

 marked in appearance and not difficult usually to distinguish from 

 the felsites, I have not been able to trace them across the whole of 

 the area ; they show vesicular structure only very sparingly, and I 

 have not recognized fluxion-structure in them. 



One rock different from any yet noticed deserves mention. It 

 was cut into at the north-western corner of the excavation for 

 the filter-beds, and a rock exactly identical with it was noticed 

 100 yards south-west of Warren House. It is a dull-white granular 

 rock, speckled with soft greenish needles and spots. Under the 

 microscope the matrix is seen to consist of quartz-granules, decom- 

 posed felspathic matter, and grains of magnetite. In this lie long 

 needles of a soft greenish decomposition-product, and large crystals of 

 decomposed felspar, some of which are triclinic and some may be 

 orthoclase. The line joining the two points where this rock was seen 

 runs a little north of west. It is probably a dyke. Mr. Harker 

 looks upon the rock as a much decomposed mica-lamprophyre. 



The large amount of calcite in many of the Warren House 

 rocks led me to keep a look-out for limestone. The following are 

 the only cases in which I detected rocks that in the least deserve 

 this ' name ; partial analyses were made of them for me by 

 Mr. R. Hornby. 



A calcareous nodule was noticed in one of the beds cut through 

 in the pipe-track. It effervesced briskly with cold acid all over, 

 and contained 56-40 per cent, soluble in cold dilute hydrochloric acid, 

 chiefly calcium carbonate. The microscope shows that, besides the 

 numerous veins of calcite, the whole rock is permeated by what 

 appears to be that mineral ; but there are angular grains of quartz, 

 and some perhaps of felspar. The rock in which this nodule lay 

 has a matrix, which may be isotropic, thickly set with a tangled 

 mass of hairs and grains of doubly-refracting mineral, and con- 

 taining blebs of quartz and some larger crystals of felspar ; it is 

 thickly veined with calcite, but effervesces only along the veins. 



A similar nodule was noticed in a bed in the cutting for the diverted 

 road. It contained 38-57 per cent, of soluble matter, nearly all 

 calcium carbonate. The microscopic slide shows a mass of broken 

 felspar-crystals, chips of quartz, and patches of a green decomposition- 

 product, the interstices between these being filled up mainly by 

 calcite. It has the look of a decomposed tuff permeated by calcite. 



In both these cases it was perfectly clear that we were dealing 

 with a single isolated nodule, and that there was no bed of lime- 

 stone, not even a string of nodules. They would seem to be cases 

 in which calcareous matter has been introduced into a rock and 

 aggregated round certain centres. Some large calcareous nodules of 



