Lituminoid Cement of the Caithness Flagstones, = 325 
beds come in, we have, it seems to me, evidence of a tem- 
porary return to humid conditions, which were followed, 
when the Strathmore Sandstones were in course of forma- 
tion, by arid conditions once more. Reasoning on the same 
lines, we may also conclude that the commencement of the 
period when the Orcadian rocks were formed was one in 
which arid and sub-arid climatal conditions prevailed. On 
the other hand, the bituminous nature of the cement of the 
Caithness Flagstones, properly so called, appears to me to 
warrant the belief that humid conditions, favourable for 
the growth of vegetation, followed the arid conditions by 
which the earlier part of the period was characterised. The 
red beds overlying the Caithness Flagstones suggest yet 
another return to conditions unfavourable to the development 
of vegetable life. These and other mutations of climate of 
a similar character finally gave way to those under which 
the varied and abundant flora of the Carboniferous Period 
was ushered in. 
Reverting to the Caithness Flagstones, we find that some 
points of importance still remain to be considered. To 
understand these, we must now review some further chemical 
reactions arising from the peculiar combination of conditions 
that usually obtain in inland lakes. One of the most im- 
portant of these relates to the presence in river-waters, and 
hence also in the lakes, of solutions of sulphate of lime. 
This, on coming into the presence of decomposing organic 
matter, and especially matter of vegetable origin, undergoes 
reduction, with, finally, the liberation of the lime in the 
form of carbonate, which may go down in such a form as to 
remain in a state of diffusion throughout the strata forming 
at the time, or else may, eventually, separate into concre- 
tions. Very few river-waters are free from some carbonate 
of magnesia, especially in the case of rivers draining areas 
in which rocks containing the ferro-magnesian silicates 
occur. Hence dolomite may be formed. The same may 
happen even where the disintegration of the ferro-magnesian 
silicates is effected by the alternate expansion and contrac- 
tion due to great diurnal ranges of temperature, such as 
usually prevail where the percentage of aqueous vapour 
