Geology. 85 



varieties. The weathering of the granite rocks to a great depth 

 is one of the marked features of the area. In the Laverton belt 

 the ore occurs as hematite-quartz lodes and as reefs ; the Burt- 

 ville mines are in small quartz reefs of exceptional richness, and 

 the Erlistoun belt contains ore in veins and mingled with hema- 

 tite-bearing quartz lodes. 



5. Geology of the Marysville Mining District, Montana / by 

 Joseph Barrell. Prof. Paper No. 57, U. S. Geol. Surv., pp. x, 

 178, with 16 plates and 9 figures, including maps in cover. As 

 stated in the sub-title, this is chiefly " a study of igneous intru- 

 sion and contact metamorphism." The Marysville region lies 17 

 miles northwest of Helena, and its importance as a mining dis- 

 trict is due to the deposition of ores in the contact-zone of a 

 small bathylith or stock of quartz diorite about 2-| miles long by 

 1^- broad at its widest part. The map which accompanies the 

 work, on a scale of £ mile to the inch, presents the detailed geol- 

 ogy of an area about 5% by 8 miles in length and breadth. The 

 sedimentary rocks are upturned limestones and shales of Algon- 

 kian age, through which the igneous mass has been intruded 

 with peripheral sheets and dikes of aplite, microdiorite and dio- 

 rite-porphyry, with some small intrusions of gabbro in another 

 place. All of these varied features receive adequate treatment, 

 but the special character of the work lies in the detailed investi- 

 gation of the bathylithic body, of the method of its intrusion, 

 of its form and of its relations to the surrounding rock masses 

 both past and present. Added to this is a study of the contact 

 phenomena produced, from various points of view, all of which 

 are treated in detail. The presentation of the geological facts 

 shown by the very careful and complete study of the district is 

 accompanied by a very full discussion of them on the theoretical 

 side, so far as they have a bearing on the main problems of the 

 area. 



In regard to the manner of intrusion, Professor Barrell believes 

 this was accomplished, not by the filling of a predetermined 

 chamber, nor by marginal assimilation and melting of the walls, 

 nor by laccolithic invasion, but by magmatic stoping, that is by 

 a process of passive invasion of the sediments, by the breaking 

 off of blocks from the roof and their subsidence into the body 

 of the magma, the latter thus eating its way upward. He is led 

 to this view by the insufficiency of other hypotheses and by cer- 

 tain positive facts which are presented. This view, as a method 

 of explanation for the "mise en place" of many bathylithic 

 bodies of granite, has been advanced by other geologists, but 

 this is the most complete and adequate presentation of it, 

 founded upon details of actual occurrence carefully worked out, 

 that has yet appeared. Thus the work, though devoted to the 

 local geology of a small area, is of wide interest since the 

 problems treated are of great general importance. 



l. v. P. 



