272 Mr. W. M. Holmes's Glauconite Casts 



of the jaw of a plesiosaurus, or long-necked sea-lizard, and also 

 an elephant's tooth ; but the celebrated elephant bed(?) lies 

 further west, which I was unable to explore. 



The western boundary of the Isle of Purbeck is Luckford lake 

 and stream ; the latter rises in the grounds of Lulworth Castle, 

 and flows into the Frome river, thus bringing us to the point 

 from which we started. 



87. — Glauconite Casts from Godstone Firestone. 



By W. Mukton Holmes. 



(Read September 10th, 1890). 



I THOUGHT it would be interesting to the members of this Club, 

 who took part in the excursion on the August Bank Holiday, if 

 a short account were given of the microscopic structure of the 

 firestone rock, quarries of which occur near Godstone, and were 

 visited on that occasion. 



In December, 1886, Dr. Hinde read a very complete paper on 

 the same subject, so complete, in fact, that I find, on comparing 

 my own notes with it, he has already forestalled all I have to 

 say ; but there are doubtless some present who have not seen the 

 original paper, and it will be well to re-state some of his con- 

 clusions : — 



" Firestone belongs to the upper greensand formation, and 

 is well shown between Godstone and Merstham. When first 

 quarried it is soft and earthy, but becomes much harder by 

 exposure to the air. It varies in colour from a whitey-brown to 

 a bluish tint, but under certain conditions it occurs as a soft 

 cream-coloured powdery material, very light, and throughout 

 filled with minute tubes. This kind is more particularly shown 

 at Farnham, and the nearest equivalent to it is at Merstham in 

 the yellowish decayed rotten-stone. The cavities are really the 

 negative casts of sponge spicules, that is, the spicules have been 

 dissolved, and only their minute impressions in the soft matrix 

 remain. This matrix is nearly entirely composed of minute 

 discs or spherules of soluble silica, and seems therefore to have 

 been derived from the solution of the spicules." 



Dr. Hinde found spicules representing each of the four prin- 

 cipal groups into which sponges are divided. After treatment 

 with acid (which in my particular specimen caused brisk 

 effervescence, indicating the presence of lime carbonate), and 

 subsequent washing, the particles are ready for mounting for 

 examination. The most conspicuous objects in the field are 

 numerous short rod-shaped bodies of an emerald-green colour. 



