ROCKS. — These are the aggregates of one or more minerals, forming part of the lithosphere as independent geological units.
They may originate either from the consolidation of natural molten masses called magmas (igneous rocks) or from the deposition of materials in suspension or in solution (sedimentary rocks) or from the transformation of igneous or sedimentary rocks (metamorphic rocks).
Igneous rocks are distinguished, on the basis of their mode of occurrence and formation, into intrusive (plutonic, abyssal) rocks, hypabyssal rocks (mostly dyke rocks), and volcanic or effusive rocks. The most modern classifications of igneous rocks, due to A. Johannsen, E. Tröger, P. Niggli, and C. Andreatta, are based on the quantitative criterion of the percentage of characteristic minerals present.
Sedimentary rocks may form from the deposition of fragments of pre-existing rocks or from the deposition of dissolved substances or from the accumulation of inorganic remains of organisms; thus there are clastic or mechanically formed rocks, chemically formed rocks, and organogenic rocks.
The complex of actions which determine the mechanical fragmentation of rocks and the chemical transformations from which the solutions or colloidal suspensions originate is called weathering: this is due largely to atmospheric agents, but in many cases the materials destined to form sedimentary rocks may also be formed by endogenic actions.
Metamorphic rocks, also called crystalline schists because of their prevailing structural pattern, represent the product of transformation and recrystallization, with possible phase changes, of igneous (orthoschists) or sedimentary (paraschists) material under altered physico-chemical conditions. The transformation is assisted by temperature, pressure, and mechanical actions.
In close relation to the genetic process, rocks exhibit characteristic structures. Thus intrusive rocks mostly show granular structure; effusive rocks may show porphyritic or vitreous structure; sedimentary rocks show clastic, granular, saccharoidal, or oolitic structures. Metamorphic rocks display a great variety of structures, including in this term also the meaning of texture: typical are granoblastic structures and the parallel structures with oriented elements of metamorphic rocks due to dynamic metamorphism or tectonites.
---
ROCCO. — In the early Church, the rocchetto (rocchetto) was a white linen vestment worn by the Roman Pontiff when he celebrated Mass in the papal chapel. It was also worn by bishops and abbots in certain circumstances, and in Rome it was sometimes worn by the Pope himself (Caerem. episc., I, 1, cap. 1, n. 2). In the Middle Ages, until the 13th century, the rocchetto was an unadorned tunic reaching to the heels and fastened at the waist; from the 14th and 15th centuries it began to be shortened, and by the 16th–17th centuries it reached just above the knees, omitting the belt. In the 18th century it barely covered the hips but was richly adorned with lace or pleats. The term "rocchetto" appears in Rome as early as the 14th century.