Vol. 51.] ON THE OCCURRENCE OF RADIOLARIA IN CHALK. 601 



Surrey gravel-pits. 1 Forms of Astromma, Haliomma, Podocyrtis, 

 and others occur ; these also, through the kindness of Prof. Rupert 

 Jones, we have had the opportunity of seeing, and they are all 

 silicified tests, many of them in good preservation. 



Several hundred slides cut from almost all the various divisions 

 of the Chalk, from many different localities, have been examined by 

 us, but nothing ever presented itself in any of these which was un- 

 mistakably a radiolarian test. Many years ago, however, one of us 

 had noticed that sections through nodules from the Melbourn Rock 

 always contained spherical bodies which had a peculiar aspect, and 

 did not occur in any other kind of Chalk. Being rather incon- 

 spicuous and apparently structureless objects, they somehow escaped 

 attention when our joint paper on the Melbourn Rock 2 was being 

 written, and no mention of them was then made. 



A few years ago we had the good fortune to obtain and examine 

 a large series of radiolarian earths, chalks, and siliceo-calcareous 

 deposits from Barbados ; a study of which made us familiar, not only 

 with the forms of radiolaria as seen in a thin rock-slice, but also 

 with the changes in structure and mineralization which they undergo 

 during the consolidation and partial alteration of the deposits after 

 these have been elevated and exposed to the action of percolating 

 water. 



Happening recently to examine some slides of Melbourn Rock 

 again, we were struck by the resemblance of the spherical bodies 

 above mentioned to the form and general appearance presented by 

 calcified and semi-destroyed radiolaria in some of the Barbadian 

 rocks. A careful comparison confirmed the fact of this resemblance, 

 and an examination of other slides disclosed the existence of forms 

 to which the spinous processes were still attached, so that we 

 became convinced of the radiolarian origin of the bodies before us. 



Finally some of the slides were submitted to Dr. Hinde, who 

 writes that he agrees with us as to the nature of most of them, and 

 though he at first suggested that some might possibly be diatoms, 

 he subsequently writes that as the spined forms are almost certainly 

 radiolaria it is most likely that the smooth circular bodies also 

 belong to that group, and he would so consider them unless flattened 

 circular Coscinodiscus diatoms were proved to be present. 



II. The Chalk in which the Radiolaria occur. 



The bodies which we take to be radiolaria were first noticed in 

 the Melbourn Rock, and since our attention has been recalled to 

 them we have found them in the nodules of samples of Melbourn 

 Rock obtained from many parts of England. They do not occur in 

 the coarser shelly matrix of that rock, but only in the nodules or 

 lumps of hard fine-grained white chalk which make up so large a 

 portion of the rock. 



The nodules of the Melbourn Rock have been described by us in 



1 Ann. & Mag. Nat. Hist. ser. 5, vol. xii. (1883) p. 52. 



2 Quart. Journ. Geol. Soc. vol. xlii. (1886) p. 216. 



