150 
Review of Recent Geological Literature. 
afterwards and now in the Museum of Science and Art at Edinburgh con¬ 
clusively proved that he was right. 
Dr. Traquair then discusses at great length the structure of Coccosteus 
and Homosteus demonstrating their close relationship and concludes by 
naming the species of which he gives a figure, H. milleri. 
Prof. T. McK. Hughes, of Cambridge, records the discovery of other 
fossils in the Lower Cambrian rocks of North Wales, in the great slate 
quarries at Penrhyn. They are very imperfect but appear to be the casts 
of the carapace of a trilobite of nearly the same size and outline as Cono- 
coryphe viola , the species recorded in 1888 from these slates. 
Prof. Hughes enters into a consideration of the possible causes which 
led to the preservation of fossils in a single spot or in a few isolated spots 
in so vast a mass of unfossiliferous slate. For it should be mentioned 
that although these quarries have been worked for many years and are 
among the most extensive in the British Isles yet no fossils had been pre¬ 
viously repotted from any part of them. 
He thinks that the slates before metamorphism may have been affected 
by a “fault and fold” disturbance whereby certain parts may have been 
caught and preserved from the intense pressure to which the rest of the 
rock was exposed and which has produced its perfect slaty cleavage and 
in so uniform a mass has totally effaced all pre-existing structure. 
Mr. Harker calls attention to the importance of pressure considered by 
itself as an important factor in metamorphism. “Many geologists” he 
says “require of mechanical force nothing except the liberation of heat by 
the crushing of the rock-masses.” But he insists that pressure alone is 
of great importance by its direct effects on chemical action even unaided 
by induced heat. He divides metamorphism accordingly into thermo¬ 
metamorphism and dynamo-metamorphism, and points out four sets of 
conditions which may be expected to govern the process in different 
places. These are 
1. Low temperature and low pressure. 
2. High temperature and low pressure. 
3. Low temperature and high pressure. 
4. High temperature and high pressure. 
He points out that a distinction between these different conditions may 
explain the frequent occurrence of certain minerals such as andalusite, 
garnet and idocrase with stratified rocks that have been altered at high 
temperatures, while other changes accompany the metamorphism of 
similar rocks at low temperatures unless the conversion of pyroxene into 
amphibole, of plagioclase into saussurite, of potash-feldspar into white 
mica and quartz, and of titaniferous iron ore into sphene. 
As to the action of water in the process he regards the water as itself one 
of the minerals involved, and remarks that its increased solvent power 
under increased pressure must lead to solution where the pressure is 
greater and deposition where it is less. 
Mittheilungen aus dem mineralogishen Institut der Univerisitdt Kiel. 
Band 1, Heft 1. Dr. J. Lehmann issues the first of a proposed series of 
publications by the Mineralogical Institute of the University. Among the 
