122 ANALYSES OF MAGNESIAN LIMESTONE. 
XXITI.—Analyses and Description of Magnesian Limestone, from 
the Trow Rocks. By Rost. Catvert CLapuam. 
Read December 23, 1861. 
At our Last Fietp Meerine, held on the 18th of October last, 
at Marsden Rocks, a discussion arose on walking along the 
Coast, as to the cause of some very marked and distinct changes 
or decompositions in the Magnesian Limestone, which are 
seen most clearly at the Trow Rocks, where the quarrying is now 
going on. The beds of limestone worked in the quarries belong 
to the Middle Bed—the Concretionary, or Cellular limestone of 
Sedgewick,*—and appear here to consist of two main divisions. 
The upper bed is dark coloured, compact and crystalline, (No. 1) 
and the lower bed is of a light cream colour, (No. 3) much softer 
than the upper bed, and can easily be cut with a knife, and is non- 
crystalline. These beds again are divided into two other beds, 
one lying immediately below the upper and crystalline bed, and 
consisting of a soft clay-like limestone, (No. 2) which can be 
easily moulded with the hands, and is of a rusty colour, and 
saturated with water. The other bed (if bed it can be called, 
as it is very irregular), consists of a conglomerated limestone, 
which is rather curious, as both from appearance and from ana- 
lysis, I find it to consist of the hard crystalline limestone of the 
upper bed, and pieces of the soft cream-coloured limestone of the’ 
lower bed intermixed with it. These cream-coloured pieces of 
limestone are subject to easy decomposition from causes which 
will be explained, and in many cases appear as a soft, whitish 
clay, and in other cases are entirely removed, and leave 
hollows in the rock of various shapes and sizes. By exposing 
the upper (crystalline bed) to water, even for some weeks, it is 
not at all acted on, but remains perfectly close and hard; but 
when the lower bed (cream-coloured and non-crystalline) is ex- 
posed to water for even a few days a change takes place, and it 
begins to crumble into a fine soft mud. This is more rapidly 
effected when the water is slightly warmed. The water which 
" * See Notes on the Permian system of Northumberland and Durham. By Richard 
Howse. Transactions, Vol. III., p. 235. 
