236 On Mineral Veins. 
that the gniess and schistose rocks were merely changed 
granites. | 
The presence of organic remains in the schistose rocks 
indicated that this could not be the case, but the investiga- 
tions of the English Geological Survey have shown that the 
observations of Mr. Hopkins were correct, although the con- 
clusions he drew from them will have to be reversed. 
The opinion that granite is only a changed form of the 
schistose rocks is daily becoming more general among prac- 
tical geologists. Messrs. Hicks and Salter, in a report on 
the geology of St. David’s, Pembrokeshire, read at the last 
meeting of the British Association (1866), state “that the 
Harlech group has a passage downwards into the central 
syenitic mass, so distinct and gradual as to induce the belief 
that that mass is throughout no other than altered Cam- 
brian.” Professor Hitchcock also describes an extensive hed 
of partially metamorphosed conglomerates where the quartz 
pebbles had been elongated, and in some cases had assumed 
a laminated structure, the elongation and lamination being 
in a meridional direction 
Whether such is the origin of the granites or not, 1t must 
still be allowed that the slow and gradual changes at 
present in operation which, in the long course of ages have 
produced the great alterations in the physical geography 
of the earth’s surface now observed, may have produced 
a like change in the rocks themselves, and unless we 
give due weight to the accumulative action of these small — 
changes (7.e. small in a limited space of time), little progress 
will be made in explaining the phenomena attending the 
formation of mineral veins. | | 
The silurian and other rocks, besides the planes of deposi- 
tion, commonly called bedding or sedimentary planes, are — 
traversed by vertical cleavage planes, and these are again — 
crossed by joints or divisional planes at different angles, — 
which penetrate to a great depth. The ordinary rock-joints — 
are usually confined to the bed in which they occur; itis — 
not these, however, that we have to consider, but only the — 
large master-joints of a district which traverse all the beds. — 
Although the direction of these divisional planes is variable, 
yet series of them running on parallel lines are found in ~ 
sufficient numbers to show that they owe their origin to — 
some general cause. I have seen large areas of flat lime- — 
stone-rocks on the shores of Northumberland, where the — 
divisional planes crossed each other with such regularity — 
