Page:A Treatise on Geology, volume 1.djvu/180

 164 in sandstones they are commonly irregular; thick-bedded limestones have nodular or uneven surfaces.

The coarse sandstones (as millstone grit) frequently present oblique lamination, which, added to the irregularity of the beds, renders it often embarrassing to say what is the true dip of such rocks. (Diag. No. 45.)

The divisional structures or cracks, joints, and fissures, vary much in relation to the nature of the rock— its fineness or coarseness of grain, the thickness or thinness of its beds, and the position of the point with regard to axes of elevation and perhaps other causes.

In the accompanying diagram, L may represent limestone., P plate, G gritstone. The joints in L are generally rectangular to the bed (in thin-bedded limestones L', the joints are more numerous).

In plate they are often oblique to the bed; in gritstone less regularly formed, being mostly cracks: this is especially the case where the beds are thick. The principal fissures F, which sometimes go through many beds, are most open and regular in the limestone.

Coal has sometimes joints of the same kind, (called