(a) Typical ophitic “plates” are not very common in the slides. More frequently the habit of the pyroxene is semi-ophitic.
(b) This character is as a general rule associated with the plexus or non-fluidal arrangement of the felspar lathes.
(c) The felspar-lathes are nearly always large, frequently averaging more than ·2 mm. in length. This coarse doleritic groundmass is almost diagnostic of an ophitic rock.
It is not always possible to allow for the influence of locality in drawing up such a classification as this, since it is well known that in each volcanic region the rocks have a particular facies recognisable in hand-specimens as well as in the slide, though it is not easy to express such a distinction in a definition. Perhaps this is represented in “adaptation” as we find it in the organic world, and the question arises as to the value of such characters for critical purposes. Regional variation plays such an important part that it cannot be ignored in rock-classification.
SYNOPSIS
I. Class.—Olivine Rocks (Plag., oliv.).
| Sub-class. | Division. | Order.[[108]] | ||||
|---|---|---|---|---|---|---|
| 1. No groundmass (non-matr.) Limburgites, Peridotites, Olivine Gabbros, etc. | (Not represented in collection.) | |||||
| 1. Felspar lathes not in flow-arrangement (non-flu.). | ||||||
| 1. Olivine abundant (cop.) | ||||||
| 2. Felspar lathes in flow-arrangement (flu.). | ||||||
| 2. Groundmass (matr.) Olivine-basalts | ||||||
| 3. Felspar lathes not in flow-arrangement (non-flu.). | ||||||
| 2. Olivine relatively scanty (pauc.). | ||||||
| 4. Felspar lathes in flow-arrangement (flu.). | ||||||
II. Class.—Augite Rocks (Plag., aug.).
| Sub-class. | Order. | ||
|---|---|---|---|
| 1. No groundmass (non-matr.) Gabbros, in part. | Not represented in collection. | ||
| 2. Groundmass (matr.) Basaltic Andesites; Augite-andesites; Pyroxene-andesites in part. | 1. Felspar lathes not in flow-arrangement (non-flu.). | ||
| 2. Felspar lathes in flow-arrangement (flu.). | |||
| 3. Felspars short and broad (orthophyric, orth.).[[109]] | |||
| 4. Groundmass felsitic (fels.)[[109]] | |||