(_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.
I. CLASS.—OLIVINE ROCKS (_Plag., oliv._).
+----------------+------------------+---------------------------------+ | SUB-CLASS. | DIVISION. | ORDER.[108] | +----------------+------------------+---------------------------------+ | 1. | | | | No groundmass | | | | (_non-matr._) | | | | LIMBURGITES, | |(Not represented in collection.) | | PERIDOTITES, | | | | OLIVINE | | | | GABBROS, | | | | etc. | | { 1. | | | | { Felspar lathes not | | |{ 1. | { in flow-arrangement | | |{ Olivine abundant | { (_non-flu._). | | |{ (_cop._) | { 2. | | |{ | { Felspar lathes | | 2. |{ | { in flow-arrangement | | Groundmass |{ | { (_flu._). | | (_matr._) |{ | | | OLIVINE-BASALTS|{ | { 3. | | |{ | { Felspar lathes not | | |{ 2. | { in flow-arrangement | | |{ Olivine | { (_non-flu._). | | |{ relatively | { 4. | | |{ scanty | { Felspar lathes | | |{ (_pauc._). | { in flow-arrangement | | | | { (_flu._). | +----------------+------------------+---------------------------------+
+----------------------+----------------------------------------------+ | SUB-CLASS. | ORDER. | +----------------------+----------------------------------------------+ | | | | 1. | | | No groundmass | Not represented in collection. | | (_non-matr._) | | | GABBROS, in part. | | | |{ 1. | | |{ Felspar lathes not in flow-arrangement | | |{ (_non-flu._). | | 2. |{ | | Groundmass (_matr._) |{ 2. | | BASALTIC ANDESITES; |{ Felspar lathes in flow-arrangement | | AUGITE-ANDESITES; |{ (_flu._). | | PYROXENE-ANDESITES |{ 3. | | in part. |{ Felspars short and broad (orthophyric, | | |{ _orth._).[109] | | |{ 4. | | |{ Groundmass felsitic (_fels._)[109] | +----------------------+----------------------------------------------+