Geological Society of London. 331 
The mineral replacements which liave affected the siliceous 
skeleton of Siplionia were then considered : in specimens preserved 
in phosphate of lime from the Gault of Folkestone the spicules 
have undergone a replacement by calcic carbonate, while tliose from 
the greensand of Haldon and other localities still possess a siliceous 
composition, though the interior of the spieules has been dissolved 
away so as to enlarge the axial quadriradiate canal to a surprising 
extent; and the silica so dissolved has been re-deposited on the 
exterior of the spieule, so as to fi.ll up the interstices of the net- 
work, and in some cases the cavities of the canal System of the 
sponge. Thus, to some slight extent, these specimens may be said 
to have fossilized themselves. 
Choanites wa6 shown to be the deep-sea form of Siplionia, the 
latter characterizing Greensand deposits, which were laid down 
in depths eorresponding to those in which existing Lithistids now 
flourish, while the former is characteristic of the chalk which was 
deposited in a deeper sea. 
The paper concluded with a systematic description of the genus. 
3. “ On the Serpentine and Associated Kocks of the Lizard Dis- 
trict.” By Rev. T. G. Bonney, M.A., F.G.S., Fellow and late Tutor 
of St. John’s College, Cambridge. 
The author stated that considerable doubt appeared still to exist 
as to the true relations of the Lizard Serpentine and the associated 
hornblende sehists; and as to the origin of the Serpentine. He had 
carefully examined all the junctions aecessible on the Cornish coast 
(inland they are generally obseured). Some of them are concealed 
by debris, etc. ; but the majority prove beyond doubt that the Ser- 
pentine is intrusive. Furtlier, almost everywhere large fragments 
of hornblende schist are caught up and ineluded in the Serpentine. 
Besides the Serpentine there is a large mass of gabbro at Crousa 
Down, and many dykes and veins along the east coast almost to 
the extremity of the Serpentine region. At Coverack Cove, near 
the above mass, are gabbros of two ages, the older much resembling 
a kind of troktolite. On microscopic examination it proves to be 
chiefly plagioclase felspar, augitie minerals (including diallage), 
and olivine partially converted into Serpentine. There is a i - ed and 
a green variety. The newer, a coarser variety, appears to be of the 
same age as the other veins on the coast, and connected with the 
main mass. Some remarkable changes have taken place in this 
also. In certain places it exhibits a Separation of its mineral con- 
stituents, causing it to resemble a foliated rock. This is proved to 
be due to pressure at right angles to the structure. The minerals 
also are often changed. The felspar is replaced by a white granulär 
mineral resembling saussurite ; the diallage (which occurs some- 
times in very large crystals) is often partially, or even wholly, 
converted into rather minute crystalline hornblende. In these 
specimens there is no olivine to be distinguished. The great mass, 
liowever, is rieh in olivine, yet a weathered specimen from it, 
resembling in aspect the gabbro of the veins, does not show olivine. 
Hence the author believes that in certain cases the olivine, instead 
