For the common metamorphic minerals the series is as follows: Titanite, rutile, hematite, ilmenite, garnet, tourmaline, staurolite, kyanite, epidote, zoisite, pyroxene, hornblende, dolomite, albite, mica, chlorite, talc-calcite-quartz, plagioclase, orthoclase, micro cline. In general, this crder is one of increasing molecular volume and decreasing density. Any particular member of this series is able to assert its own crystal form against that of any other member following it in the series. Where crystal form is developed, only facies with simple crystallographic indices are developed, and these are commonly cleavage facies. Hornblendes have promi nent development of (Ho) faces, garnet (Ho), kyanite (loo) and (oio). Crystals showing a well developed form are known as idioblasts, and those characterized by absence of crystal form xenoblasts. Where the stresses developed by the growth of crystals in a solid rock are not offset by stresses imposed from without, being in random directions they tend to be neutralized by mutual compensation. The directionless hornfels structure of contact rocks is produced in this way, but the various schistose and foli ated structures are the result of the imposition of shearing stress from without by which the internal stresses are reduced.
It is to be expected, therefore, that those minerals which occur as idioblasts in metamorphic rocks are members high in the crystalloblastic series—as rutile, garnet, etc. They are known as strong minerals, in contradistinction to weak minerals, such as quartz and the potash felspars. The structures given by inclu sions, relative form, development and size of crystals, have thus no direct analogy with the apparently similar features observed in igneous rocks. Whereas, in the latter, inclusions are older and tend to have better crystal shape than their host, in metamorphic rocks both host and inclusion may be developed simultaneously, and not infrequently inclusions are xenoblastic and the host idio blastic. The porphyritic crystals of igneous rocks are an early generation, but in metamorphic rocks the corresponding large crystals are pseudo-porphyritic. They are known as porphyro
blasts. Frequently they are strong minerals, as garnet, ilmenite, hornblende, etc., and though formed at the same time as the ground mass minerals, they are not infrequently aligned across the common schistosity or foliation planes. Many porphyroblasts are characterized by multiple twinning (kyanite, chloritoid, cor dierite), or show the typical sieve structure, being crowded with the ground-mass constituents (cordierite, chloritoid, staurolite, chiastolite) ; again, many of them are unrepresented among the ground-mass minerals. The large size of porphyroblasts is, doubt less, in large measure to be accounted for by their relatively greater crystallization force and crystallization velocity. Indeed, it is not improbable that many of them grow rapidly from super saturated solutions; their twinning inclusions and orientation point to rapid growth. While for the elements of the ground-mass, nuclei are usually originally present, with porphyroblasts it is frequently otherwise ; garnet, staurolite and chloritoid are new phases developing in a medium free from the crystal nuclei of these minerals. In the absence of the latter, spontaneous crystal lization from supersaturated solutions is rendered possible. In the highest grades of metamorphism porphyroblast structure is less in evidence than in medium grades, owing to a tendency to wards equalization of the size maxima of different minerals under very high temperatures and pressures. At the same time sieve structures tend to be abolished. Where the structures of the original rocks are incompletely effaced during metamorphism, they are known as relict or palimpsest structures. Some amphibolites may thus preserve the porphyritic or ophitic structures of the dolerites from which they are derived, conglomerate-schists or quartzites the psephitic or psammitic structures of sediments. These relict features are usually referred to by prefixing the term "blasto" to the specific structure, as blastophitic, blastopsephitic, etc.