Yol. 65.] THE BOULDERS OF THE CAMBRIDGE DRIFT. 253 



isotropic mineral with two cleavages at 71°, and a very low refractive 

 index. This we identify as fluorspar, and the identification is 

 confirmed by Dr. Bonney. 



Rocks clearly referable to the Central Yalley of Scotland and the 

 Cheviot distriet are not common in the immediate neighbourhood 

 of Cambridge, but a few specimens have been found, including 

 North Berwick orthophyres, aud Carboniferous or Old Bed Sandstone 

 lavas, and both dyke and lava-facies of the Cheviot complex. Some 

 large boulders found in a pit on the Huntingdon road, near the 

 Traveller's Best inn, may perhaps be referred to the teschenite- 

 group of the Edinburgh district. 



There were found in the Sedgwick Museum two specimens of 

 much significance ; the labels are almost illegible, but they appear 

 to read ' Observatory, Cambridge,' and ' Drift, Cambridge,' re- 

 spectively. Both of these bear a very strong resemblance to certain 

 varieties of the Borrowdale lavas, and if they had been found in 

 the North of England they would have been assigned without 

 hesitation to this source. If this determination is correct, their 

 occurrence is of the highest interest. 



One specimen of a porphyritic rock with large felspars shows a 

 very strong resemblance to some of the acid apophyses of the Shap 

 and Eskdale granites. This must be considered in connexion with 

 the occurrence of lavas of Borrowdale type just mentioned. 



Carboniferous sediments, including both the Limestone and 

 the Millstone Grit, are fairly common ; while many sandstones 

 were undoubtedly derived from the Coal-Measures. Two specimens 

 of highly characteristic cellular Magnesian Limestone are also 

 recorded. Jurassic sediments are plentiful, and there is in our 

 collection one specimen from Milton Boad of a peculiar silicified 

 oolite, which has a wide distribution in the Cambridge Drift : its 

 origin has not yet been determined. Carstone, Hunstanton Bed 

 Bock, and hard, pale, pink Chalk are common to the east and south 

 of Cambridge, but not on the north and west ; they are always 

 associated with specimens of Gryplicea dilatata bored by Pliolas, 

 and the distribution of this group is of great importance in de- 

 limiting the boundaries of different ice-streams, as will appear 

 later. 



Besides these determinable types a large number of specimens 

 of acid and basic igneous rocks, together with gneisses and schists, 

 are found, as to the origin of which it is impossible to say any- 

 thing definite. Many of these have been examined microscopically, 

 and from this examination at least one fact is clearly apparent, 

 namely, that the majority of the granites and gneisses are derived 

 from an alkaline petrographical province ; in nearly all cases the 

 dominant felspar is some form of perthite, and soda-bearing ferro- 

 magnesian minerals are equally common. This suggests, at all 

 events, a strong probability that many of them are of Scandinavian 

 origin. Throughout the district dolerites and basalts, both with 

 and without olivine, are very common. "We have examined many 



