﻿Geological Society of London. 331 



The mineral replacements wliicla have affected the siliceous 

 skeleton of SipJionia 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 those from 

 the greensand of Haldon and other localities still possess a siliceous 

 composition, though the interior of the spicules lias 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 spicule, so as to fill 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 was shown to be the deep-sea form of Siplionia, the 

 latter characterizing Grreensand deposits, which were laid down 

 in depths corresponding 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 Sei-pentine and Associated Eocks of the Lizard Dis- 

 trict." By Kev. 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 schists ; and as to the origin of the serpentine. He had 

 carefully examined all the junctions accessible on the Cornish coast 

 (inland they are generally obscured). Some of them are concealed 

 by debris, etc. ; but the majority prove beyond doubt that the ser- 

 pentine is intrusive. Further, almost everywhere large fragments 

 of hornblende schist are caught up and included 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, augitic minerals (including diallage), 

 and olivine partially converted into serpentine. There is a red 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 granular 

 mineral resembling saussurite ; the diallage (which occurs some- 

 times in very large crystals) is often jDartially, or even wholly, 

 converted into rather minute crystalline hornblende. In these 

 specimens there is no olivine to be distinguished. The great mass, 

 however, is rich 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 



