
ON AN EXCAVATION AT COLLYHURST. 943 
The different beds will be described in the descending order :— 
Ist, The Till— Throughout Lancashire this deposit is known in the country 
by the name of marl, and near towns as brick-clay, and in an economical point 
of view is the most valuable deposit in the drift series. It is composed of a 
stiff brown clay, mixed with a little sand, and containing a small proportion of 
carbonate of lime. On being treated with hydrochloric acid, it effervesces with 
considerable briskness. The clay when cut down with a knife presents a brown 
colour, but when allowed to cleave in the open air, which it will do both ver- 
tically and horizontally in the act of drying, the sides of the cleavages are 
coated with a covering of dull blue colour, probably arising from the pre- 
sence of a small quantity of carbonate of iron. In it are mingled without 
any order of position blocks of red and light-coloured granites, sienites, por- 
phyries, greenstones, basalts, and various other rocks of igneous origin, clay 
slates, Silurian rocks, mountain limestones, cherts, millstone grits, all the in- 
durated rocks, ironstones, and coals of the carboniferous series, magnesian 
limestones, and waterstones (the last-named lying i situ amongst the upper 
red marls), but no other rocks of a more modern date have as yet been found 
in it. These fragments are of various sizes, from the size of an ordinary pea 
to blocks of five tons in weight, and lie mingled together without any order 
of deposition. 
The external characters of the boulders in the till are remarkable ; some 
present well-rounded surfaces, others having lost the angles on one or two 
sides, the edges of the remaining sides are quite sharp; some are scored with 
striz and polished, and many are quite angular, as if they had been recently 
separated from their parent rocks, having scarcely undergone any attrition. 
The granites, porphyries, greenstones, and all the hard slate rocks, none of 
which now occur 7m situ within a less distance than near 100 miles, are for 
the most part, but certainly not all of them, well-rounded, and many of them 
are marked with striz on one or both of their sides. Rocks at present found 
wm situ nearer Manchester, such as mountain limestones and cherts, are more 
angular and less striated and polished than those last mentioned, but the mill- 
stone grits, carboniferous strata, magnesian limestones, and waterstones, al- 
though some of them are striated, polished and rounded, have generally sharp 
edges. 
Boulders in the till at Collyhurst near the edge of the great fault there 
are both more numerous and larger than those found in the same deposit in 
any other place around Manchester. A well-rounded block of greenstone, 
three tons in weight, was found some years ago in Mr. Buckley’s delph, and 
since the last meeting of the Association at Manchester, a block of millstone 
grit, of between four and five tons in weight, has been met with in the same 
place. Three of the sides of this last-named mass have lost their edges, while 
the remaining one bears evidence of having undergone considerable friction. 
It has been stated that the boulders occur in the clay without any distinct 
lines of deposition, mingled together pell-mell. This is the case at Collyhurst, 
but still there are at one point distinct lines of a regular deposition from water. 
At about a yard from the bottom of the till occurs a deposit of fine laminated 
silt, something resembling the warp of our English rivers, with thin layers of 
fine sand: it is from ten to twelve inches in thickness. This bed is quite 
free from pebbles, although the till both above and below it is full of them. 
Generally speaking the silt lies level, but there are several places where it is 
seen much contorted and twisted without the underlying lower new red 
sandstone exhibiting any corresponding appearances. The total thickness of 
the till is twenty-one feet, and it rests on an uneven surface of lower new 
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