1835.] Geological Sketch of the Neilghcrries. 417 



Immediately below the vegetable soil, in almost all places, we find 

 a stratum of detritus (in general not above a few inches thick), which 

 is different in different localities, according to the nature of the rock 

 on which it rests. Thus, it is ferruginous on those places where 

 iron ores are found : quartzy and silicious above the thick veins of 

 quartz, which intersect these rocks. But in general it is composed 

 of small fragments, sometimes rounded, and sometimes angular, of 

 the decomposed rock (of which we shall speak hereafter), being iden- 

 tical with that we see on the surface of the soil (No. 7). 



The simple inspection of this detritus, overlying, and correspond- 

 ing in position and nature to the subjacent rock, forces upon us the 

 conclusion, that it does not belong to the alluvium (terrains de trans- 

 port), but that it has its origin in the disintegration of the rock in situ, 

 without any, or any material displacement from the rock which has 

 given rise to it. 



Another fact that proves this detritus to arise from the decomposition 

 of the underlying rock, previous to its becoming lithomargic earth, 

 and while in the dry friable state which seems to have preceded it, 

 is, that the porcelain earth, wherever this last earth is found in large 

 beds below the vegetable earth, is never overlaid with detritus ; 

 because the rock is all at once converted into porcelain earth, without 

 the intermediate passage into the dry friable rock, from which the 

 detritus arises. 



This detritus is seen almost in all localities on these hills ; the 

 numerous sections that have been made in their declivities, for the new 

 roads, show it clearly every where. On looking at the banks on the 

 sides of those sections, we observe the detritus adapting itself to all 

 the irregularities and zig-zags of the subjacent rock, or stratum. 

 Fig. 2 of PI. XXXI. shows this conformity better than any descrip- 

 tion. It is taken from the bank of the road round the lake near the 

 bund. 



That this detritus has not been transported from any distance is 

 further proved, by observing it on the surface of the soil in those 

 places where the protruding rocks are either decomposed or decom- 

 posing. We often see the still undecomposed nucleus of the rock 

 protruding through the soil, surrounded and enveloped by the numerous 

 concentric layers of the decomposed rock, the bassets of which we see 

 level with the soil, the upper portion of them having been disinte- 

 grated into a detritus, which is scattered on the soil in the vicinity of 

 the blocks. As far as I know, no organic remains have hitherto been 

 found either in this detritus, or in the black soil. 



In some places the detritus, for causes difficult to guess, assumes 



