6 THE RHATIC BEDS. 
containing a less amount of ferric oxide. Like the Red Marls, 
they are occasionally found containing pseudomorphs of rock 
salt, and this fact, coupled with the entire absence of phosphates 
indicates the same phase of sedimentation as that of the Red 
Marls. The Green Marls, in fact, represent the last phase of 
deposition in the land-locked and highly saline seas of the New 
Red period. But those waters, which had been veritable Dead 
Seas for ages, were once more on the point of teeming with life. 
In a space of time, which must have been very short compared 
with the rate of sedimentation, the barriers which had kept back 
the tidal waters of the ocean from the great salt-lakes were broken 
through, and the comparatively fresh water, charged with its 
living freight, terminated for ever the New Red epoch. We are 
accustomed to regard all geological changes which have not been 
caused by volcanic action as having taken place gradually and 
with extreme slowness, and doubtless this is as a rule correct. 
But we have here, I think, most unmistakable evidence of a very 
rapid and extraordinary change in physical conditions, and I am 
somewhat surprised that this should have hitherto escaped due 
attention. It is probably owing to the fact, to which I have 
before referred, that the Rhzetics have for the most part been 
studied in coast sections, and not in newly-cut faces. In the 
former case, of course, natural weathering would preclude any- 
thing like a minute study of the line of junction. 
It will be an interesting point to ascertain if the black shales 
differ in composition from the underlying Tea Green Marls in any 
other respects than in the organic matter, phosphates, and pyrites 
which they contain. From a rapid examination of samples of 
each, from just above and below the junction, I am inclined to 
think they show a great resemblance to each other, and if a 
quantitative analysis should prove this, it will go far to show, 
what is extremely probable, that the streams entering the open 
Rheetic Sea still continued to bring down the same kind of sedi- 
ment as they did at the close of the Triassic period. 
I have still to describe the upper part of the Wigston section, 
which I will do as briefly as possible. The black shales as they 
