BATHOLITH, in geology, a term applied to a large mass of intrusive igneous rock usually showing transgressive relations to the enclosing country rock (from Gr. (3aO15s, deep, and a stone). The name was first used to describe those largest of all intrusions, characteristically developed in great mountain ranges, and thus includes those masses known as "central granites," "in trusive mountain cores," etc. Batholiths are distinguished from stocks only in size, the latter term being applied by many writers to irregular masses of batholithic habit with an area of outcrop not exceeding 1 oo square kilometres. Some of the best known batholiths reach thousands of square kilometres in area, such as those of Alaska, British Columbia and Patagonia. The discordant relations of these intrusions are shown by the rupture of the bedding or other structural planes of the enclosing wall rock. Great masses of the rocks invaded are often floated off and are found lying in the heart of the consolidated magma much trans f ormed by the heat to which they have been exposed, and tra versed themselves by the magma in ramifying threads. There can be little doubt that batholiths have developed during or closely following great orogenic movements; most are elongated parallel to the tectonic axis of the folded belts in which they are, and many show a primary gneissic banding clearly indicative of flow movement during the epoch of crystallization.
Two views are now prevalent as regards their mode of origin. According to the one, batholiths are conical masses rising from great depths and assimilating the strata which lie above and around them. This process is facilitated by the mechanical dis lodgement or stoping of great masses of the enclosing rock which sink through the magma and are dissolved at depth. Thus inter preted, batholiths are active agents in making room for them selves by stoping and assimilation. Chemical no less than struc tural relations with the surrounding rocks preclude acceptance of this view: there is an entire absence of evidence for the existence of the excessive heat required for large scale assimilation processes, also the chemical homogeneity of those portions exposed to view is in itself a strong ground for its rejection.
The opponents of this theory hold that stoping or mechanical dislodgement is merely a subordinate factor in the emplacement of batholiths. Of late years much detailed study has been made of the granite batholiths of Silesia, the principal results of which we owe to Cloos and his co-workers. A field study of these in trusions (such as the massifs of Lausitz, Riesengebirge, Striegau and Strehlen) shows that they are injected between the country rocks and are therefore not bottomless. The magma coming from a deep seated source has ripen and spread in different directions, the amount and direction of horizontal spreading being largely de termined by folds, faults or other lines of weakness in the surrounding rocks. Space is thus obtained by thrusting aside, lifting or even forcing down the adjacent rocks during the fold ing movements. Conclusions respecting the shape of batholiths have been obtained from numberless observations on the linear elongation of the component minerals (the "stretching"), dispo sition of the joints and their variable pitch and dip.
These features within any particular massif exhibit in general a dome-like arrangement, the underlying source of the magma being found beneath the crest of the dome. By these means the Hauzenburg massif, for example, is shown to consist of an elon gate arch with roots on the north-east side and a nearly level floor on the south-west ; others are shown to possess a sill-like structure, the magma spreading laterally from a source situated along a line of weakness such as an overthrust. A considerable majority of batholiths are constituted of acid igneous rocks, generally of granitic, quartzdioritic or granodioritic composition. Large batholiths of basic composition are rare. Of such, those built up of anorthosite, e.g., in eastern Canada and New York State, are best known.