Cheris and Dolomites. Be} 
proceeding process of chemical alteration and substitution 
in place, producing all the varieties of product according to 
the original mineral composition of the material and the 
extent to which the process has progressed. The original 
condition of the rocks would seem to have been that of a 
series of sands and clays laid down in the basin of deposition 
in the order of their coarseness. The sands would natur- 
ally preponderate in the areas of shallower water, becoming 
gradually mixed and interspersed with clayey matter, and 
the latter entirely replacing the former in passing towards 
the areas of greater depth. 
The percolation of chemical waters subsequent to the 
elevation of the area and the compacting of the rocks would 
then produce all the varied phenomena noticed. ‘The sandy 
parts being easily permeable would be most changed, whilst 
the impermeable clays would be left in their original con- 
dition, and the sandy clays would form an intermediate 
product. 
The original grains of the sands having a very varying 
composition, would by their decomposition afford all the 
different materials at present constituting the rocks, the 
ferruginous elements being supplied where basic silicates 
constituted the grains. 
The examination of these rocks by the microscopic 
method further reveals the nature and extent of the process 
of alteration above outlined, the following descriptions of 
three thin sections by Mr. W. 8. Bailey ' giving a very 
good idea of the most noticeable characteristics. 
“No. 281. R. 64. Locarton (Chert).—This rock is com- 
posed in greater part of what were originally round and 
irregular pieces of felspar, in a ground mass of quartz. 
The felspar has for the most part been entirely replaced by 
its various decomposition products, viz., calcite, chlorite 
and hydrated iron oxides. That portion which has not 
undergone this alteration has been completely replaced by 
silica; so that round, cloudy areas of silica (principally in 
the form of chaleedony) now appear where originally 
1 Annual Reports Geological Survey, Vol. III, pp. 120 an 122 H. 
