THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. VIII. 

 No. I.— JANUARY, 1901. 



o:EiXG-Ti<rj^Xj JLT^TIC^LES. 



L — Note on the Structure of Sarsens. 



By rrofessor J. W. Judd, C.B., LL.D., F.R.S., V.r.G.S., etc. 



[^Introductory Note. — After the publication of my paper on the 

 Sarsens, or Sarsen Stones, in the Wiltshire Archifiological and 

 Natural History Society's Magazine, vol. xxiii (188G), pp. 122-154, 

 many friendly communications gave me further information on the 

 subject, and additional references to published facts and opinions. 

 From this correspondence, and my own notes made in the countr}', 

 I propose to utilize much that seems to be of interest. The most 

 important of these additions to our knowledge of the Sarsens is the 

 following memoir on their constitution and structure by my fi-iend 

 Prof. Dr. J. W. Judd, C.B., F.R.S., etc., of the Royal College of 

 Science, who most obligingly examined with care the microscopical 

 structure of manj' specimens from authenticated localities. With 

 his kind pex'mission this valuable communication (dated March 9th, 

 1888) is here printed. — T. Eupert Jones.] 



THE microscopic examination of a series of thin sections, cut 

 from the Sarsens, shows that their minute structure varies as 

 strikingly as does the appearance of their fractured surfaces. 

 Microscopically, the Sarsens are seen to be made up of two kinds 

 of materials, clastic fragments of crystalline minerals and a cement 

 (base or matrix) of a microcrystalline or cryptocrystalline character. 

 The relative proportion of these two constituents varies very widely 

 in different cases. 



The Sarsens with saccharoid fracture stand at one end of the 

 series. An admirable example from Camberley, North Surrey, is 

 seen to be almost wholly made up of sand grains, with very little in 

 the way of cement visible. Much of the cementing material in this 

 rock is ferruginous, and the rock is more incoherent than is the case 

 with most Sarsens. 



At the other end of the series stand the Sai'sens exhibiting 

 a fracture resembling that of some cherts. Under the microscope 

 the greater part of their mass is seen to be made up of excessively 

 minute and imperfectly developed quartz, microlites, and these 



I)EC.\UE IV. — VOL. VIII.— NO. I. I 



