Alfred Harker—Physics of Metamorphism. 15 
There is first the accumulation in the hollows and an overlap of the 
later over the younger part of the same series as the successive 
deposits creep up the flanks of the submerged land. 
And, secondly, there is the exaggeration of the thickmess of the 
mud accumulated in the hollows when pressure supervenes and 
squeezes it up against the base of the cliffs and steep mountain 
chains of the ancient submerged land. 
Thus the manner of occurrence of the fossils in the Penrhyn 
quarry throws some additional light on the processes which have 
modified the older rocks even to the exaggeration of thickness. 
There is not such a great difference in the thickness of the lowest 
part of the series at Bethesda and Bangor, and even that difference 
can be much reduced by allowing for the increase in the apparent 
thickness of the Penrhyn slates by squeeze. 
The still greater missing series at Carnarvon may be accounted 
for by the overlap of higher over lower Cambrian against an 
important sea-bordering Archean mountain range. The lowest 
Cambrian beds at Carnarvon were thus a higher and newer part of 
the system, and being nearer the tops of the mountain ridges 
escaped the great crush that caught the older and deeper beds 
thrown down nearer the base of the ancient range. 
IIJ.—Nortes on tHe Puysics or MrTamorpHism. 
By Atrrep Harker, M.A., F.G.S., 
Fellow of St. John’s College, Cambridge. 
HE problem of the metamorphism of rock-masses is one in 
which increased study has not led to unanimity of opinion. 
The literature of the recent Geological Congress in London suffices 
to remind us, how widely divergent are the conclusions to which 
various geologists have been led by researches in the field. Since, 
then, the a posteriori line gives such very different results in different 
hands, it may be worth while to revert for a moment to the deductive 
method, and try to trace the consequences, in this connexion, of 
admitted physical principles. 
Since some definition of our subject is necessary, we will, for the 
present purpose, group together under the name metamorphism all 
processes which result in a partial or complete crystallization or 
re-crystallization of solid masses of rocks. Such changes are usually 
effected through chemical re-arrangements, and are often attended 
by the development or creation of special structural planes in the 
rock-masses. ‘Those processes which do not appear to demand 
either a high temperature or a high pressure, such as the conversion 
of sandstone into quartzite by deposition of interstitial quartz, 
although logically included here, will not be discussed. We may 
conveniently distinguish them as hydro-metamorphism, recognizing 
the important part played by water in changes of this kind. 
The geological manuals lay down a fundamental distinction 
between contact- and regional-metamorphism ; but from our present 
starting-point we arrive at a somewhat different division. It is 
