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which undoubtedly has occurred-would give rise to gypsum. The presence of the marl and salt, I hope to show, offers no objection to this explanation; but simply accounts for the aluminous silicates of the alkalies which existed in the original rock masses.?
The facts obtained so far in the examination of these rocks confirm the conclusions suggested by the quartz-crystals ; and both agree with the evidence obtained in the field. It would not be surprising to find on more careful search in this area minerals containing boracic acid.
Summary of conclu. From the study of the specimens the following consions.
clusions have suggested themselves to me :1. That the gypsum-masses are not of aqueous or sedimentary origin. 2. That they are formed by the addition of water to anhydrite. 3. That the quartz-crystals were formed at a period anterior to the hydration,
but after the formation of the anhydrite. 4. That the anhydrite may have been produced by the action of sulphuric
acid on limestone, the action probably taking place at a high temperature and in the presence of super-heated waters, in the manner in which many igneous rocks have been altered by solfataric action subsequent to their primary consolidation.
EXPLANATION OF PLATES.
Plate I. Fig. 1. Section of “gypseous” matrix of the bi-pyramidal quartz-crystals. Com
posed of anhydrite exhibiting pinacoidal cleavage and lamellar twinning (in two crystals); and gypsum in one large crystal ophitically developed around the anhydrite with repeated twinning. From Márí, North-West
Punjáb. Magnified x 143 diameters. (See p. 241.) Fig. 2. Horizontal section of a bi-pyramidal crystal of quartz, showing anhydrite
included in small crystals which are, towards the edge, arranged in
zonal lines. From Márí. Magnified x 143 diameters. (See p. 232.) Fig. 3. A rock composed of gypsum and anhydrite, the latter in small crystals drawn
out to form a schistose structure. From the Mayo mines, Khewra, (so
called “semi-anhydrite ”). Magnified x 143 diameters. (See p. 241.) Fig. 4. Section from the same specimen as Fig. 1. The lamellæ of gypsum are
Plate II. Fig. 1. Crystal of anhydrite, the fragments of which have suffered a slight angular
displacement (24) by the development of gypsum between the pieces. From the matrix containing bi-pyramidal crystals of quartz, Márí.
i It is interesting to note that Mr. Mallet has long ago suggested that the gypsum of the Spiti valley has been due to thermo-aqueous agencies (Mem , Geo. Surv., Ind., vol. v: (1866), p. 157). I have already shown that the earlier stages of hydration of the sulphate of lime are present in the massive anhydrite collected by Mr. Mallet in the Spiti area (ante, p. 240).
· The presence of the Khewra trap in the salt-marl remains as the solitary proof that undoubted igneous action has taken place in this area.
Magnified x 310. (See p. 241.) . Fig. 2. Crystal of anhydrite showing cleavage and twinning. T, T' and T" are
twinned patches in which the cleavage lines and optical characters are at an angle to those in the general mass of the crystal. The directions of these lines are shown in the diagram (Fig. 3). From massive anhydrite,
Spiti valley, North-West Himalayas. Magnified 310. (See p. 241.) Fig. 3. Diagram showing the directions of the cleavage and twinning lines of the
specimen shown in Fig. 2.
OA, directions of pinacoidal cleavage. OD=most perfect, lying
most perfect) corresponds to OD of the remainder of the
to OA (least perfect). The angles are shown in the figure. Fig. 4. Crystal of anhydrite showing the two directions of twinning with gliding
planes produced by pressure the centre of which coincided with A. From the massive anhydrite of Spiti, North-West Himalayas. Magnified x 310. (See p. 241.)
GEOLOGICAL SURVEY OF INDIA DEPARTMENT.
No. 9.-ENDING 31ST OCTOBER 1891.
Director's Office, Calcutta, 31st October 1891.
P. N. Bose, B.Sc., 2nd Grade Deputy Superintendent.
Fritz NOETLING, Ph.D., Palæontologist.
P. N. Datta, B.Sc., Assistant Superintendent.
W. B. Dallas EDWARDS, A.R.C.s., Assistant Superintendent.