30£ 
Dr Hibbert on the Distribution 
small processes from the epidotic sieiiite, by invading the strata 
attached to it, disturb their direction, whilst the strata again ren- 
der irregular their mutual line of connection, by penetrating more 
or less into the substance of the sienite, that we must have re- 
course to other observations than those afforded at the actual 
junction, in order to consider whether strata viewed en masse 
are joined by their sides to an unstratified rock, coating or sur- 
rounding it after the manner of the layers of an onion, to which 
they have been often compared ; or if, on the contrary, they are 
opposed to the rock by their lateral edges in an unconformable 
position. This is best inferred by a comparison of the general 
direction assumed by the strata, and the outline of the unstrati- 
fied mass which is opposed to them. The direction of the line 
forming the southern boundary of the epidotic sienite is about N. 
W. whilst that of the strata meeting it is about N. 15® 
E.; consequently the bounding line of the sienite cannot be con- 
formable to the strata, or, in other words, parallel to, or collate- 
ral with them, but must rather be opposed to the lateral edges 
of the strata, and thus be unconformable to them. 
The southern boundary of the sienite may be traced from a 
small island in Quendal Bay, named' Little Holm, where it is 
joined to a conglomerate rock, to be noticed hereafter; from 
thence to Cross Island and the Mainland. Passing close to the 
house of Mr Ogilvie, it is with difficulty traced through a cover- 
ed country until seen at Noss, connected with gneiss and mica^. 
slate in the manner above mentioned. The northerly line of 
boundary, with other particulars relating to the sienite, I shall 
point out on another occasion. One important circumstance, 
however, in the sienite, remains unnoticed. This concerns the 
deep form of the rock where it affords an attachment to strata, 
and which is disclosed by a deep section on one occasion only^ 
namely, at Noss. 
Every unstratified mass, in its junction with strata, may ex- 
hibit to us different descriptions' of surface. For if a vertical 
line passing through the highest point of the unstratified rock, 
should not fall within its base, such a surface of the rock, may, 
for the sake of an important distinction, be described as impen^ 
dent OY hanging over. But if the perpendicular, on the con- 
trary, be included within the base, the rock may be considered 
