PART IV. GREENSTONE ROCKS, & c . CHAPTER I. DIORITE, DIABASE, MINETTE, &c. Generally called Greenstones. The rocks which come under the above heads cannot be considered as generally suited for archi tectural, and only exceptionally for ornamental pur poses, and consequently might have been passed over in silence in this work, which properly extends only to rocks of this description. In adopting this course, however, an hiatus would have occurred un expected by some readers, and inconvenient to the student; and I therefore propose to give a short account of the origin, nature, and uses of a group of rocks which is very largely distributed over the earth’s surface. Origin. Though greenstones and basalts are both of igneous origin, and similar in general appearance, yet they may be considered as the representatives of two great classes, differing in composition and gene-