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It will be noticed that in each case the water actually found was less than the calculated percentage. This is probably due to impurities in the specimen. The same remark applies to the results of all other cases enumerated below.
In a similar manner I have investigated the specimens collected from the Mayo mines and containing, according to Dr. Warth's analyses, 5 per cent. of water (“ semi-anhydrite”).
Five fragments were cut from different parts of the specimen (M. 2627), and the specific gravity of each fragment determined. One, possessing a medium specific gravity, was selected for chemical analysis. In the remainder only the water was estimated
(1) Specific gravity : 2-523.
Chemical composition :
( Magnesia . .
A mixture of gypsum and anhydrite with a specific gravity of 2'523 possesses theoretically a composition of Mineralogical
By comparing these results with those obtained by analysis it will be found that the differences are no greater than can be accounted for by the presence of impurities.
The following results were obtained for the remaining fragments :
Nos. 2 and 3 are examples of the wide variation in composition in different parts of the same hand-specimen. By Mohr's method I determined the specific gravity of the whole specimen and found it to be 2.472. From this specific gravity one may, with a fair degree of accuracy, estimate its mineralogical and chemical composition thus:
A second specimen from the same locality and labelled also “semi-anhydrite" offers evidence of the same nature (No. 1).
A chemical analysis gave the following results :
Specific gravity: 2.511.
a result not widely differing from the analysis given above.
The specific gravity of the large hand-specimen determined by Mohr's method proved to be 2 523. The average mineral composition, therefore, is approximately
These results differ very widely from those obtained by Dr. Warth; but I am unable to find any source of error which can account for the very great discrepancy
The carbonic acid existed in very small quantities, and its actual estimation would be of little value compared to the additional time it would occupy.
between our analyses. Dr. Warth has unfortunately omitted to give the details or number of the analyses by which he obtained an average of 5 per cent. of water in these rocks. In order to investigate still earlier stages in the hydration of calcium-sulphate, I
made a similar examination of the massive anhydrite colAnhydrite from Spiti. lected by Mr. F. R. Mallet in the Spiti valley, North-West
Himalayas. Portions of these specimens contained cleavable anhydrite, which on analysis confirmed Mr. Mallet's determination, cleavage-fragments giving an average specific gravity of 2949.
Two fragments, however, selected from the granular part of the specimen showed the earlier stages of hydration. Having determined the specific gravity of both pieces I estimated the water in one and made an analysis of the other.
The first piece had a density of 2.846 and contained 3.37 per cent. of water. The calculation from the specific gravity gives a composition of
The specific gravity of the second fragment was 2.832, and on chemical analysis I obtained the following result :
Water . . . . . . . . 3066
. . . . . 39'59 Ca SO, = 95-93.
• • • • • • tr.
results very closely agreeing with those obtained on analysis. The close agreement existing between the specific gravity and chemical com
position in each case leaves little doubt as to the nature of Microscopic structure.
these rocks, but the results are confirmed in the most striking manner by the microscopic characters of thin sections.
On account of the solubility of gypsum in water I at first attempted to cut the sections in oil; but this substance would not permit of the reduction of a rock so soft and friable to sections sufficiently thin. At last I found the use of a concentrated solution of sulphate of lime allowed of the most satisfactory results.
Since writing the above, Dr. Warth has very kindly offered to send me his typical specimen to which Mr. Wynne referred as "semi-anhydrite." On its arrival, I hope to subject this specimen to a similar investigation as to its chemical and physical characters; the results will appear in the next number of the Records,
A large number of sections examined under the microscope show every gradation from the massive part of Mr. Mallet's Spiti specimens, in which large crystals of anhydrite are cemented with mere films of gypsum polarising with characteristic colours of the lower orders, through others in which the gypsum assumes a larger proportion and ultimately makes up the entire field.
In the earlier stages hydration has developed gypsum along cleavage-cracks and twinning planes. In the specimens from the Mayo mines gypsum stretches across the field in large irregular plates, including numerous crystals of anhydrite, and exhibiting a constancy of orientation over considerable areas. This structure at
once recalls the ophitic structure so common in the doleritic Ophitic structure.
rocks, although it is entirely different in origin, being, in this rock, a secondary structure due to hydration. The gypsum-crystals sometimes exhibit lamellar twinning running across these ophitic plates and regardless of the included anhydrite (Plate I, Figs. 1 and 4). The clear gypsum and the bent lamellæ forcibly recall in appearance the fresh plagioclase-felspars of igneous rocks (Plate I, Fig. 4). The hydration of sulphate of lime to gypsum is accompanied by an expan
sion in the mass of nearly 30 per cent. These changes Expansion of the mass.
masse can be followed in the microscope. Cases are found in which the formation of gypsum in a cleavage-crack has given rise to a slight angular displacement of the cleaved fragment (Plate II, Fig. 1). Faulting of twinning and
cleavage-planes arises from the same cause. These changes Schistose structure.
• can be traced to a rock in which crystals are fractured and the fragments scattered along lines to produce a distinctly foliated structure—a selfinduced schistosity-a structure observable even in the hand-specimens collected from the Mayo mines (Plate I, fig. 3). The expansion which has produced such changes in the internal structure of these rocks must have been an important factor in the causes which led to the stratigraphical disturbances in the super-incumbent beds. The anhydrite occurs in crystals in which two pinacoidal cleavages are always
noticeable, one frequently more perfect than the other. In Microscopic characters of anhydrite.
thin slices the colours are always bright and seldom as low
as the first order. With convergent polarised light the bi-axial figure is often obtained, but the wide optic-axial angle prevents a reliable determination of the dispersion. The results I obtained appeared to be p <v. The positive character of the double refraction is easily determined.
A noticeable feature in all the specimens is the distinct lamellar twinning, which Twinning of anhydrite.
appears to be parallel to the brachydome of Dana, 17.1
nco (See Plate II, Figs. 2 and 3.) The regularity of the cleavage planes and twinning is shown in the case figured, in which the two sets of twinning planes are easily noticeable. The pressure to which the rock has been subjected and the consequent production of gliding planes in the crystals is illustrated in Plate II, Fig. 4. The centre of crushing seems to have been at the point A, where an irregular mosaic of broken crystal-fragments has been produced, and the crystal has yielded along the gliding planes shown in the sketch.
i Dana reads the axes in a different order to that adopted by Rosenbusch. According to the former author the acute bisectrix is perpendicular to the basal plane and the optic-axial plane lies in i-i.
Taken with the chemical and other characters the microscopic examination of
these rocks leaves little doubt that the anhydrite has, by the Hydration of anhydrite.
de absorption of water, recrystallized as gypsum. These specimens do not offer the faintest evidence in favour of the notion that a compound between Ca SO4 and Ca SO4, 2 H,O exists with specific characters.
It would not be surprising to find that many of the large masses of the so-called anhydrite-rock of Nova Scotia, Switzerland and other places prove on microscopic and chemical examination to be mixtures of sulphate of lime, anhydrous and hydrous. Professor How, in an interesting series of papers on the mineralogy of Nova Scotia, considers that the masses of anhydrite-rock exposed in the Gut of Canseau, between Nova Scotia proper and Cape Breton, have originated independently of the associated gypsum, and doubts the formation of gypsum from anhydrite.
K. von Fritsch, however, has shown that the gypsum of Airolo and Val Canaria has been produced from anhydrite with an accompanying increase in bulk and consequent disturbance of the adjacent rocks, in which minerals as hard as quartz are crushed and bent by the pressure. There is no doubt, I think, that the gypsum of the Salt Range in areas so widely
..separated as Khewra and Márí as well as at Kálábágh, west Origin of the Salt Range gypsum.
of the Indus, has been the result of the alteration of anhy
drite ; and the production of this latter mineral by deposition from water at ordinary temperatures cannot be conceived to be possible. The evidence, therefore, of the laboratory is distinctly against the supposition that the gypsum in these areas, at least, has been produced by the evaporation of water containing sulphate of lime. That this conclusion applies in general to the whole of the gypsum-deposits in the Salt Range I do not assert; but it is worthy of remark that the localities have in no way been selected and are widely separated. This conclusion, moreover, coincides precisely with the facts obtained by Mr. Middlemiss during a recent examination of the rocks in the field, and recorded by him in the paper already quoted.3 Mr. Middlemiss, whilst denying the aqueous origin of the salt-marl, suggests that it might be traced to the action of subterranean forces, the precise nature of which the facts so far obtainable in the field are insufficient to determine.
Concerning the gypsum of Nova Scotia, Sir William Dawson has suggested that its origin might be due to the action of volcanic waters containing sulphuric acid on limestones; but he cannot thus account for the intermixed anhydrite. Professor How's discovery of boron-bearing minerals in these rocks confirms, in his opinion, Dawson's theory.5
There seems to be no reason why a somewhat similar action might not have taken place on masses of limestone in the Salt Range area. Under pressure and at a high temperature anhydrite would be produced, and subsequent hydration of the mass