From Henry Clifton Sorby   March 27/57.

Broomfield, Sheffield | March 27/57.

My dear Tyndall,

I am extremely obliged to you for your kind note,¹ but am sorry that circumstances will prevent me seeing you as you name. It was all arranged for us to leave here this week, and I should then have been with you before now; but we were compelled to put off till next Monday, when, after staying a day or two with David Forbes² in Birmingham,³ we shall proceed to Cornwall⁴ and shall return by London towards the end of May. I am sorry I could not see you, because I much wished to explain to you some new principles of research which I have been working at the last half year, and are such as I think will much interest you, and will lead to some very striking results. However to partly make up for not being able to explain them personally I will just give you an outline of them; for I should much like you to know of them, and they have never been published, and but few know of them.

When crystals are formed from any solution, it very often happens that portions of it are caught up in the crystal, and may be seen with a high magnifying power. Now suppose the liquid was water or an aqueous solution, and the temperature elevated, there would be a cavity containing water and which by contracting or becoming cool would leave a vacuity or bubble. The appearance then is thus [image here]. When such are very small it often happens that the bubble moves about, apparently spontaneously, in a very curious manner. If strongly heated, crystals containing such fluid cavities, fly to pieces or loose the water more gently, and then none is seen and no bubble exists in the cavities. But if the crystal had formed from a fused glassy solvent, instead of water being caught up, small drops of this glass become enclosed in the crystal, and, on cooling more, the glass often contracts and forms a spherical vacuity, which of course never moves about and still remains after strong heating. In other cases, on the cooling of the glass, small crystals are deposited from it in and on the sides of the cavity; thus in glassy feldspar⁵ These two kinds of cavities, viz. aqueous and glassy, are characteristic of crystals formed from solution or from fusion—artificial crystals deposited from water having cavities with water and those formed from fusion in slag &c. containing glass &c. We therefore have here a most excellent guide in studying mineral crystals in rocks, whether they were formed from solution in water or solution in glass—whether they be of aqueous or igneous formation.

Now you will at once perceive that I have thus got hold of a most excellent method of research, applicable to the study of rocks, and I will just give you a slight outline of what I have made out so far.

A power of 400–800 liniar⁶ is required. You perhaps are aware that the igneous origin of some volcanic minerals, even of [leucite]⁷ has been questioned.⁸ When such are however examined, from Vesuvius,⁹ they are seen to contain no aqueous cavities but glassy, passing into crystalline, just as in the crystals formed in slags;¹⁰ whilst the calcareous spar¹¹ of Vesuvius contains aqueous cavaties, indicating an aqueous origin. In the same manner when their sections of greenstones¹² &c are examined they show most admirable examples of glass cavities, but no aqueous, except in some of the zeiolites¹³ found in them by aqueous action after the partial cooling of the previously melted rock. We can thus clearly determine which minerals were formed by the cooling of the fused rock and what afterwards produced by aqueous metamorphism. So far as I have explored, basalts¹⁴ and greenstones, and such rocks have a structure that closely agrees with modern volcanic products and artificial slag and which proves most conclusively that they were once in a fused condition but have certainly been very greatly altered by subsequent aqueous action.

Now applying the same principles to granitic rocks,¹⁵ which differ so very much from any modern volcanic product, what is the result? Probably the feldspar has been formed from fusion, but the quartz (the mineral whose presence distinguishes the rock so much from modern lavas,) is full of cavities containing water—it has every appearance of the quartz of veins, which nearly every one believes to have been formed by aqueous action and, so far as I at present can see, there is no good reason to come to any other conclusion than that it was formed from solution in water. Now, unless water has some properties not now understood, (we must discuss this when I see you) the quartz must have been deposited at about 300°F, for it expands so as to fill the cavities at about that, whereas if it had been present when at a red heat, the cavities should have been only about half full, instead of about 15/16.

Now, from what I have said, you will see I have got hold of some very curious principles, which, when thoroughy worked out and applied, will unquestionably clear up some of the most difficult points in the history of rock masses. However the subject is quite in its infancy. When you examine thin sections of some aqueous rocks it appears at first as if it was a new world, and everything requires to be learned, so that, though I have even now made out many very curious facts, I am sure far more remains to be done. It is to work out this problem that I am going into Cornwall.

There are several other general principles that are of very great value in proving mineral changes, that I will explain when I see you, involving optical properties.

Yours very truly | Henry Clifton Sorby

My address will be as usual.

RI MS JT/1/TYP/4/1349–51

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