28 THE ROYAL SOCIETY OF CANADA of feet in length and generally less than 300 feet in width. Some of them consist only of schistose serpentine, others of massive ellipsoidal fragments up to 3 feet in diameter in a sheared matrix. Thin sections were cut from ellipsoids of various sizes, as well as from the sheared matrix and the schist of the non-fragmental types. In the largest and least altered ellipsoids, serpentinization has not gone beyond the chrysotile and mesh antigorite stages. In others, more altered, it has reached the bladelike antigorite and amorphous serpentine stages. In all the smaller fragments examined, the serpentinization has reached the amorphous serpentine stage. The sheared matrix between the fragments, on the other hand, is always composed of bladelike antigorite, as is the schist of the non-fragmental shear zones. In the light of the above observations, it would seem that if shearing and serpentinization were synchronous, only bladed antigorite forms, and the same is true if shearing occurred after serpentinization; whereas under normal conditions of serpentinization, chrysotile, amorphous serpentine, meshlike and bladelike antigorite may be formed. Du Rietz (1935, p. 254) in his study of the serpentines of northern Sweden has come to practically the same conclusion. A satisfactory theory of serpentinization of the ultrabasic rocks of the Fort Fraser map-area must explain the following facts: (1) The serpentinization is confined to the ultrabasic rocks. (2) The serpentini- zation of the ultrabasic bodies has a haphazard arrangement unrelated to their borders. It is as intense at the centres of the masses as at the borders. (3) The areal distribution of the zones of intensity of serpentinization is in no way related to the proximity of younger acid intrusives. If the solutions producing serpentinization came from the younger acid intrusives, hydration would be expected to increase towards the contacts but this is not the case. (4) Serpentini- zation is neither confined nor related to fracture zones although, as indicated in a previous section, antigoritization seems to be favoured by shearing. The serpentinization of peridotite has for many years been a subject of controversy, and those who have studied this problem favour either one or the other of two theories: (1) That serpentini- zation 1s brought about by hydrothermal solutions emanating from later intrusive rocks. Dresser (1913), Graham (1917), and Du Rietz (1935) advocate this cause. (2) That the serpentinizing solutions emanated from the cooling magma itself, a theory strongly supported in recent years by Benson (1918), Phillips (1927), Cooke (1932, 1937), Haapala (1936), Hess (1983), and other workers. In the Fort Fraser map-area serpentinization is not related to