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ON THE FORMATION OF MINERALS. 207 
III. InFivence oF THE FELsPATHiIc SOLUTION ON THE STRUCTURE OF SOME 
CamBrian Rocks. 
The schistose deposits of Bray Head, regarded as the lowest stratified beds 
of the Cambrian system, containing the fossil Oldhamia, considered as the 
most ancient vestige of animal life on the globe, exhibit a well-marked example- 
of felspathic metamorphism effected by the agency of water. This*rock is 
specially remarkable by the system of joints which it possesses, these joints 
separating into rhomboidal prisms, presenting the angles of cleavage of the 
orthoclase felspar, the planes of bedding corresponding to the planes of cleavage 
of the felspar. Nevertheless, as we might expect in a rock which has been 
submitted to the influence of other mechanical forces, the angles do not pre- 
sent that exactitude which a crystal of pure felspar would present. Hydro- 
chlorie acid does not alter the structure of the rock; after the action of the 
acid, it can be divided into plates as thin as paper. These plates, examined 
by the microscope, exhibit a felspathic paste in crystals often distinct, and 
enveloping grains of sand. 
We have here a felspathic solution which has modified a sedimentary rock 
containing fossils, the existence of which is not contested, and has com- 
municated to it its physical characteristics. Whatever may be the first origin 
of the felspathic solution, the rock could not be deposited except under the 
action of water, having its fossils disposed in horizontal layers. The system 
of joints which this rock presents is not a simple mechanical accident ; it is 
evidently due to the natural arrangement which the molecules of the felspar 
have assumed when deposited from the solution. It is, in fact, a simple 
phenomenon of crystallization ; that is, the jointing was guided by the planes 
of cleavage, as being the direction of least resistance. 
Microscopic examination after the treatment by acid shows almost always 
carbonaceous matter in the neighbourhood, or accompanying the prints of 
fossils, that matter being often enveloped by the felspathic paste. 
Metamorphosed Arenaceous Rocks of the same Formation.—As in the pre- 
ceding rocks, the felspathic solution has sensibly influenced the form which 
the quartz-rock affects ; the crystalline forms of orthoclase predominate at all 
points. This latter mineral has impressed its mineralogical characters on the 
rock in a rude manner, it is true, but still sufficiently sensible not to escape 
observation. 
By an analogous phenomenon to that which takes place in the sandstone 
of Fontainebleau, but in a manner less striking, the active solution percolating 
through the arenaceous matter has communicated to it its crystalline charac- 
ters. The prints of felspar, which often show themselves on the surface of 
these rocks, are sometimes identical in form and in size with the large ones 
found in the granite locality of Dalkey. 
It is not always easy to follow the transition of these felspathic rocks, and 
there is a moment when they are nearly indistinguishable from rocks con- 
sidered as granite veins. There is, in reality, no great difference between some 
of the Cambrian rocks containing a felspathic paste sensibly crystalline and 
enclosing grains of quartz, and the veins of Kurite filling the cracks and 
erevices of the Dalkey granite, this paste of Eurite often containing Garnets, 
and always isolated grains of quartz which could not be developed in it, 
Logically, I do not see why these veins should not be due to causes analo- 
gous to those which have produced the felspar of the Cambrian rocks, 
