QUEKETT'S LECTURES ON HISTOLOGY. 



81 



are twice curved, like that shown at a, but each extremity is expanded, so 

 as to resemble the fluke of an anchor ; to such form, the term hi-curvate 



A, portion of the skeleton of a siliceous sponge, b, c, e, flattened spicula. n t>, pin-shaped 

 spicula. F, tri-radiate spicula in Grantia compressa. h, granules of sand imbedded in homy 

 fibre of Dijsidea. 



ancJiorate has been given ; two of these spicula are represented at h. The 

 sponge in which they occurred, like that of the preceding, was of small 

 size, and brought from the South Seas." 



From the skeletons of Sponges we pass to those of Diato- 

 maceze, Desmidieae, Foraminifera, and Nummulites. The fol- 

 lowing observations on the structure of Oolitic rocks are not 

 perhaps generally known. 



" Before I leave this part of my subject, I must say a few words on the 

 Oolites, which were formerly supposed to consist of the remains of 

 organized beings of a globular figure, like the roe or eggs of fishes, hut 

 which are usually nothing more than grains of sand, each surrounded by a 

 globular deposit of carbonate of lime and cemented together so as to form 

 masses of limestone rock. The Oolites make up no inconsiderable part of 

 the strata of this island ; according to Ure,* they form a zone 30 miles 

 broad in England, and are divided by geologists into the upper, middle, 

 and lower Oolites. They furnish a most valuable material for archi- 

 tectural purposes ; and are exceedingly rich in fossil remains, especially 

 those of reptiles and corals. 



" The egg-like particles vary considerably in size, being in some cases 

 almost invisible to the naked eye, whilst in others they are nearly as 

 large as peas ; this last form of Oolite has received the name of Pisolite, 

 differing, however, from tlie true Oolites only in the relative size of the 

 globular concretions. Bath stone, Portland stone, and the slate of Stones- 

 field, near Oxford, are all examples of Oolite. In fig. 52, a, is represented 

 a portion of that form of Oolite termed Pisolite of its natural size ; the 



Diet, of Arts and Manufactures, Art. Oolite. 



VOL. III. 



