2 Professor J. W. Judd — Structure of Sarsens. 



occasionally exhibit a tendency to the spherulitic arrangement. 

 A beautiful example of this kind of Sarsen is one from Poxwell 

 Eing, near Dorchester. In this case the original sand grains seem 

 to have almost wholly disappeared, and an aggregate of grains of 

 secondary quartz has been formed, which crystallize out freely 

 on the sides of cavities. In parts, the section shows admirable 

 spherulitic structure, and the iron-oxides have separated into small 

 globular masses. The appearances exhibited, are strikingly like 

 those of some flints with highly crystalline structure. 



All the other sections examined show the detrital crystalline 

 particles enveloped in more or less of the fine-grained secondary 

 matrix. The detrital grains consist mainly of quartz. By far the 

 greater part of these quartz grains exhibit the bands of liquid 

 cavities so characteristic of the quartz of granites and gneisses ; 

 corroded quartz grains with glass or stone cavities, evidently derived 

 from quartz-felsites, occur, but are much less rare, as are also the 

 polysynthetic grains, some of which may have been derived from 

 schistose rocks. With the quartz grains are a few unmistakable 

 particles of flint, but these are never numerous. Felspars and other 

 minerals are usually rare. Sometimes the grains appear to be well 

 rounded, and at other times they seem perfectly angular; but it is 

 probable that in all cases a considerable amount of corrosion of the 

 surfaces of the grains has taken place. Only in one or two doubtful 

 cases have I seen what could be taken as a deposition of secondary 

 silica upon, and in optical continuity with, the detrital quartz. 



In a specimen from the valley of the Kennet (Enborne Lodge 

 gravel-pit) we have perfectly angular quartz grains embedded in 

 a nearly compact cement — one which can be resolved only under 

 very high microscopic power. 



A very remarkable variety of Sarsen is one from Staple-Fitzpaine, 

 about 10 miles west of Taunton. In this rock the grains are much 

 larger than in any other Sarsen that I have examined ; they are 

 markedly angular, and though quartz grains form a majority of the 

 whole, yet felspars and other minerals occur much more usually 

 than in the other specimens examined. If this should be found 

 to be the rule with Sarsens from the most westerly localities, it 

 would indicate that the granitic and metamorphic rocks which 

 yielded the materials of which they are composed lay to the west 

 of the London Basin. 



[In a subsequent letter (February 27th, 1S89) Professor Judd 

 states that this " specimen from Staple-Fitzpaine has a fragment of 

 whitened flint in it. The microscopic characters of which are 

 unmistakably those of a silicified Chalk-mud full of fragments of 

 Globigerina."] 



The cement of the flint-conglomerate of Hertfordshire consists of 

 quartz grains, with a few grains of flint, embedded in a crypto- 

 crystalline siliceous groundmass. There is no very striking 

 resemblance between the cement of this conglomerate and that of 

 any of the Sarsens which I have examined. 



