6f the Rocks Shetland, BIS 
i^ourse Calculated to intercept, in a mode similar to that already 
described, those of the new strata of clay-slate. 
All the strata mentioned dip to the west, the quartz, hornn. 
blende, and hmestone, at angles generally from 40° to 60° ; the 
clay-slate at about 70° 
Thus there are certain general circumstances to be observed 
in this complicated distribution, materially aifecting the general 
character of the rocks concerned in it. By the curved direc-. 
tion or the peculiar locality of the quartz, hornblende-slate, and 
limestone, an interception of the rectilinear strata of the clay- 
slate seems, in the first place, to be effected. But the assump- 
tion of a more easterly direction of the clay-slate considered en 
masse, seems to depend not only upon the interception of those 
rectilinear strata which maintain a direction that is no longer to 
be continued, but also upon the presence of other masses of 
quartz, hornblende-slate, or limestone, calculated to afford an 
attachment to new strata of clay-slate, and by their curvature 
or peculiar locality to alter the direction, and with it the dispo- 
sition of the strata intermediate to them. In the application of 
appearances like these to our speculations regarding the conso- 
lidation of the crust of the globe, the influence of the hornblende- 
slate, quartz, or limestone, apparently exerted on the direction 
of the strata of clay-slate, strongly resembles that of nuclei in 
processes of crystallization. 
Suflicient has probably been said explanatory of the impor- 
tance which I attach to the ascertainment, in every junction of 
rocks of different kinds under circumstances similar to those 
described, which of the strata are continuous, and which are in- 
terrupted ; or, in other words, which of the masses has, like a 
nucleus in processes of crystallization, an arbitrary, or which 
has a dependent form. 
Returning to the clay-slate, it may be observed, that atChanner- 
wick, an inclosed roundish mass of granite makes its appearance; 
it is probably about 120 yards in diameter. The strata are, at 
the contact, much disturbed, particularly in their inclination, 
which, instead of being at an angle of 76°, is about 23° to 25°. 
I shall now state the relations of the clay-slate to the consi- 
derable mass of granular quartz situated to the east of Shet- 
land. 
