Gabbroid Rocks of Minnesota. — Winchcll. 1 5 1 
Decadocrinus grandis W. & Spr. 
P'ig. 17. Plates of ventral sac with terminal spine, showing the 
pores at the sides of the plates, and also between the 
plates and the terminal spine f 
Scaphiocrinus arboreus Worthen. 
Fig. 18. Entire specimen, showing upper part of perforated ven- 
tral sac, anterior view — I 
I 
ji 
Fig. 19. Same specmien, posterior view — -- 
Fig. 20. Exposed part of the ventral sac shown in fig. ig, with 
the pores on the sutures at the middle of the sides of the 
plates » 
Fig. 21. The upper part of ventral sac of another specimen \ 
Fig. 22. Posterior view of another specimen, showing full length 
of tube, with pores on the suture lines at the middle of 
the sides of plates -i 
Fig. 23. Part of the ventral sac of another specimen ground off 
to half the depth of the plates, showing perforation of 
the sac by pores at the middle of the sides of the plates .... f 
MINERALOGICAL AND PETROGRAPHIC STUDY 
OF THE GABBROID ROCKS OF MINNESOTA, 
AND MORE PARTICULARLY, OF 
THE PLAGIOCLASYTES.* 
(Plates VIII to XIX.) 
By Alexander N. Winchell, Butte, Mout. 
CONTENTS. 
Introduction. 
1. Geograpliy and topography of tho region studied. 
U. Geographic distribution of the rock types. 
3. Outline of the geology of the district according to tho latest views. 
4. Correlation of the nomenclatures. 
Chapter I. Olivine gabbro. 
1. Geological occurrence. 
2. Exterior characters. 
3. Texture and mineralogical composition. 
4. Detailed study of tlie miuorals. 
5. Inclusions. 
6. Alterations. 
7. Chemical composition. 
*The facts presented in this series of papers are essentially the same 
as those which appeared in the form of a thesis, presented to the Faculty 
of Sciences of Paris to obtain the title of Doctor of the Universitv of 
Paris, and entitled: Etude mineralogique et petrographique des roches 
gabbroi'ques de I'Etat de Minnesota, Etats Unis, et plus specialcment 
des anorthosites. Paris igoo. 
