PRIMITIVE Roc Ks.—These are granite, gneiss, mica slate, clay slate, quartz, and greenstone.
Granite. This rock is composed of quartz, felspar, and mica. The quartz sometimes occurs in the form of rock crystal in clrusy cavities, and then it is accompanied with the subspecies of felspar named adularia, and cry stallized mica. The proportion of mica is small ; a cir cumstance which seems connected with the age of the granite. The granite is sometimes traversed by veins of quartz, and also of small granular granite ; and, in other instances, we observe roundish and angular cotem porancuus masses of small granular granite, included in the coarse granular granite. These masses have been confounded with fragment,, and have been adduced as a proof of the newness of the granite.
Professor Jameson seems to think, that it is stratified in some places ; but professor Playfair rather doubts the accuracy of his observations. It appears to form the base on which all the other rocks of the island rest, and hence is considered as the oldest, or first formed rock of the whole series.
2. Gneiss. This rock is composed of felspar, quartz, and mica; but it contains more mica than granite. The constituent parts are sometimes so intimately intermixed, that they are not at first discernable, and the rock ap pears simple rather than compound. It is distinctly stratified, and the strata arc very much inclined. It rests upon granite, and is covered by mica slate. It, is often traversed by coternporaneous veins of felspar,; and by similar veins of a granular granitic rock, which might be confounded with true granite. CotemporaneOus mas ses of felspar, and of granular granite gneiss, occur in form of kidney beds, of greater or less extent.
3. Mica slate. This rock is composed of mica and quartz, and, like gneiss, has a slaty structure. It is dis tinctly stratified. It rests on the gneiss, and is covered by clay slate. In some places it contains small and large granular lead-glance, (sulphuret of lead,) but in inconsi derable quantity.
4. Clay slate. This iS a simple mountain rock. It sometimes contains thin beds of quartz. It is distinctly stratified, and rests upon mica slate, and is either expos ed, or covered with particular species of fixtz rocks.
Quartz. This rock occurs principally in inconsi derable beds in the mica slate and clay slate.
6. Greenstone. This rock appears, from fragments found in Glen Rosa, to occur in beds in mica slate.
Fnz•rz Rocxs.—These arc sandstone, limestone, slate clay, bituminous shale, clay iron stone, glance coal, greenstone, porphyry slate, claystone, pitcnstone, and obsidian.
1. Sandstone. Its general colour is reddish. It al ternates from fine to very coarse granular, which latter passes to congloinerate. It rest on clay slate. Is dis tinctly stratified, and the strata, in general, are more ho rizontal than inclined.
2. Limestone. This rock has the usual characters of flRtz limestone. It occurs in beds in the sandstone.
3. Slate clay. This rock occurs in considerable beds in the sandstone, and is sometimes associated with clay iron stone.
4. Bituminous shale. This rock also occurs in beds in the sandstone.
5. Clay iron stone. Of this species both the common and columnar subspecies occur ; they are associated with slate clay, limestone, and are contained in sandstone.
6. Glance coal. It is the slaty subspecies that occurs in this island. It is accompanied with clay iron stone, limestone, slate clay, &c. all of which are included in a sandstone formation. It is of but little importance in an ceconomical point of view, because it occurs in small quantities.
7. Greenstone. This rock, which is composed of horn blende and felspar, occurs in considerable quantity in the southern parts of the island, resting on sandstone. It appears in some places in beds in the sandstone.
8. Porphyry slate. This is a compound rock, hav ing a basis of clinkstoile, which contains imbedded crys tals of felspar.
It occurs sometimes in tabular, and frequently in co lumnar distinct concretions, as in the island of Lamlash ; also very frequently in veins that traverse sandstone, greenstone, Ste. It is a frequent rock in the south part of the island, where it rests on sandstone : it occurs in smaller portions in the northern part of the island, and then it appears in veins traversing the granite.
9. Claystone. This rock occurs in beds along with the porphyry slate, as at Cory Gills.
10. Pitchstone. Tills rock is generally porphyritic ; it occurs in beds, and also in veins that traverse the dif ferent primitive and fixtz rocks. It is a frequent rock, and is found particularly abundant in the southern parts of the island. It constitutes a distinct formation ; and we owe the first account of it to professor Jameson.
11. Obsidian. This rock was first observed in Arran by professor Jameson. It has hitherto been found only in loose masses. It probably belongs to the fixtz rocks.
12. Pearl-stone. Tnis interesting mineral occurs in the pitchstone, and was discovered in Arran by Drs Ogil by and Almon.