66 Prof. Bonney, On the Microscopic structure of a [Jan. 28, 



" As is usually the case with the boulders and fossil remains of 

 the Cambridge Greensand, this specimen has upon its surface a 

 number of attached plicatulse and other small shells, and it bears 

 also two patches of the phosphatic nodules characteristic of the bed 

 from which it has been obtained, and even a fragment of the marl 

 itself. 



" While the boulder has clearly been subjected to very great 

 wear, and has the external appearance usually attributed to the 

 action of ice when found in similar boulders of more recent periods, 

 there are upon it no distinct or definite scratches or grooves. 



" Taken alone, no theory as to the prevalence or otherwise of 

 floating ice in the sea of the period during which the lower part of 

 the chalk was deposited can be founded on this particular boulder. 

 But it at all events supports the already existing theory, based on 

 the character of the boulders and pebbles already described from 

 the Cambridge Greensand. It has two characteristics of ice-borne 

 erratics : — 1. It is superficially like boulders recognised as having 

 been transported from distant sources by ice, and subjected to the 

 peculiar wear and tear incident to ice-action. % Its material is 

 derived from a parent rock which can under no probable circum- 

 stances have existed, at the period of the chalk, within a very 

 considerable distance of its recently discovered resting-place. We 

 may therefore fairly, I think, accept it as evidence of the proba- 

 bility of the existence of floating ice in the sea of the chalk period 1 ." 



The rock is a very compact quartzfelsite of a dull purple 

 colour. Examined with a hand-lens, small specks of quartz are seen 

 to be scattered in the matrix, which exhibits some faint indications 

 of a fluidal structure. The rock is singularly well preserved, the 

 purple hue predominating up to the exterior surface, and marked 

 indications of decomposition not extending inwards for more than 

 a quarter of an inch. Considering that the rock has been lying for 

 ages in a waterbearing stratum this strikes me as noteworthy. 

 The rock reminds me in general aspect of specimens which I have 

 seen in the volcanic breccias of the N. W. part of Charnwood 

 Forest, especially of a compact purplish felsite (old rhyolite) from 

 Timber wood Hill. I fully expected that there would be a close 

 microscopic resemblance. This however is not the case. I have 

 examined the chief varieties of the Charnwood fragments. They 

 generally exhibit a "devitrified" matrix without definitely formed 

 crystallites, or spherulites, and with but slight and rare traces of 

 fluidal structure; in this matrix are small scattered crystals or 

 crystalline grains of quartz and felspar. This generalization is 

 founded upon eighteen slides cut from different specimens in my 



1 See a Paper by Messrs Sollas and Jukes-Browne, Quart. Journ. Geol. Soc. 

 Vol. xxix. p. 113, 



