22 
5. The Jurassic system. These strata have not yet been fully explored, 
but in many places abound with Estheria andjCy there. In the flinty limestone 
near Eidgeway in Dorsetshire are collected specimens of Cythereis, a sub- 
genus of Cythere from which it differs by having three tubercles from 
which rises a ridge continuing round the edge of the valve. 
6. The Cretaceous system is extremely prolific in Ostracoda, as Cythe- 
rella, Cythere, Bairdia, Cythereis, Cytherea, and Cytheridea. 
7. The Tertiary system. Here we find several sub-genera that have not 
occurred before, as Cyprideis, Cytherideis, and Candoya. The middle 
Eocene deposits are so full of their remains that the clay is rendered quite 
fissile. 
Mr. Stoddart concluded his paper by exhibiting specimens of nearly 
every genus and most of the species, pointing out those localities which he 
had found most favorable to the collector. 
IV. 
Eocks: Igneous and Metamorphic. 
By W. Sanders, F.E.S., F.G.S. 
Read at the Sectional Meeting, March \2>th, 1872. 
[Abstract.] 
Mr. Sanders presented at the outset a sketch of the constituents of the 
exterior of the earth; the agencies — atmospheric, aqueous, or igneous — 
which modify the surface of the globe ; the general arrangement of the 
materials; their structure and chemical composition; their division into 
sedimentary and igneous. 
The unstratified igneous rocks were treated under the divisions Plutonic 
and Volcanic. The Plutonic rocks were shown in two groups, the Felspa- 
[ thic which contain 60 to 80 per cent, of silica, and the Hornblendic or 
Augitic with only 45 to 55 per cent, of that element. The principal rock 
of the first group is Granite, a compound of three ingredients, Quartz, 
Felspar, and Mica ; Felspar, containing potash, is called Orthoclase ; with 
soda, it is called Albite ; and when both these alkalies are present, the term 
Oligoclase is used. A mixture of Quartz and Felspar only is called Eurite 
or Aplite. 
