Page:A Treatise on Geology, volume 1.djvu/153

 CHAP. VI. whole series of argillaceous and most of the arenaceous rocks are full of laminæ of deposition: beds are very distinct in the sandstones: the limestones are also regularly stratified, though nodular and uneven on their surfaces, and sometimes partially lenticular or included among shales, like other calcareous rocks supposed to have originated as coral reefs. According to what we have found to be a general law, that divisional planes abound and are regular in proportion to the regularity of the laminæ of deposition, the argillaceous beds of this system are seen to be very exactly divided by joints and fissures, some of which seem frequently to coincide with the axis of elevation of the country (in Wales this is frequently N. E.), and others to be rectangulated thereto. In this respect, however, variations occur, so that Mr. Murchison has often found two sets of joints, both forming oblique angles with the strike of the strata. In the Wren's Nest at Dudley the long joints in the limestone) observed with care, July, 1837, appeared to me to be nearly parallel to the axis of elevation, which is N. and S., or, on the east side of the hill, N. 10° E. The joints 1) in other directions were few and irregular; and though cracks were not infrequent in the thinner layers of " Baven" (argillo-calcareous and fossiliferous), the most general direction of the long joints in this also was N. 10 E. The planes of these joints are not always even nearly rectangled to the stratification they are sometimes waving in their outline, and present other circumstances of interest, particularly striations and associated strings of calcareous spar. The faults in this hill obey the law of displacement, which is given in the "Geology of Yorkshire," and illustrated p. 40. sup.

The planes of the joints and fissures are stated by Mr. Murchison to be nearly rectangled to the planes of the strata. In the country near Llandovery the lower Silurian rocks are metamorphic slates; the slaty cleavage,