The mode in which the plutonic masses of this group have reached their present position is highly problematical and still forms the subject of a lively discussion, to which attention has already been called in another connection (see p. 291). From the purely descriptive point of view, the special characteristic of these bodies is that their diameter increases downward to unknown depths and, consequently, that they do not rest upon a floor of country rock.
Stocks or Bosses are rounded or irregular masses of intrusive rock, which vary in diameter from a few feet to several miles; they cut across the country rock, which they have sometimes pushed aside and sometimes cleanly perforated, and with which the contact is steeply inclined or vertical. The structure of the country rocks, such as bedding-planes, has no effect upon the shape of the stock. From many stocks are given off tongues or apophyses, which penetrate the country rock as veins, dykes, sills, and various irregular protrusions. Granite, diorite, and gabbro are especially common in stocks and the texture frequently becomes coarser from the circumference to the centre of the mass. In many instances, perhaps generally, stocks themselves are but protrusions from larger masses.
Batholiths are great masses of plutonic rock hundreds or even thousands of miles in extent; in general characteristics they agree with stocks, except for their very much greater size, yet small batholiths and large stocks grade into one another, so that any line of demarcation between them must be arbitrarily drawn; probably all true stocks, could they be followed down, would prove to be protrusions from batholiths. Granite is the commonest batholithic rock, and in such masses forms the core of many great mountain ranges, like the Sierra Nevada and the Rocky Mountains.