Sec. 13. Now, when these crystals or grains, represented by the black spots and lines in Fig. 3, lie as they do at _a_ in that figure, in all directions, cast hither and thither among the other materials of the stone,--sometimes on their faces, sometimes on their sides, sometimes on their edges,--they give the rock an irregularly granulated appearance and structure, so that it will break with equal ease in any direction; but if these crystals lie all one way, with their sides parallel, as at _b_, they give the rock a striped or slaty look, and it will most readily break in the direction in which they lie, separating itself into folia or plates, more or less distinctly according to the quantity of mica in its mass. In the example Fig. 4, a piece of rock from the top of Mont Breven, there are very few of them, and the material with which they are surrounded is so hard and compact that the whole mass breaks irregularly, like a solid flint, beneath the hammer; but the plates of mica nevertheless influence the fracture on a large scale, and occasion, as we shall see hereafter, the peculiar form of the precipice at the summit of the mountain.[43] The rocks which are destitute of mica, or in which the mica lies irregularly, or in which it is altogether absent, I shall call Compact Crystallines. The rocks in which the mica lies regularly I shall call Slaty Crystallines. [Illustration: FIG. 4.] COMPACT CRYSTALLINES. Sec. 14. 1st. Compact Crystallines.--Under this head are embraced the large group of the granites, syenites, and porphyries,--rocks which all agree in the following particulars:-- Their first characteristic. _Speckledness._