220 W. M. Hiitchings— Ash-slates of the Lahe District. 



of our sedimentary slates, inasmuch as a very large amount of new 

 mica has been formed, yet in the matter of the titanic acid con- 

 tained in the material acted upon the result is very different. 



It is beyond question that the volcanic rocks of the Lake District 

 contain a quite considerable amount of titanic acid, though it has not 

 been determined in the analyses made.^ 



Probably a portion was combined in the form of sphene, but most 

 likely the greater part was contained in the augitic minerals. In 

 the weathered and much altered andesites as we now have them, 

 secondary s\)\\enQ is one of the most constant constituents, in granular 

 form and also as little clusters and groups of crystals, often evenly 

 disseminated throughout the rock. This sphene has probably been 

 mainly formed during the decay of the augites and the basic portion 

 of the ground-masses. At Shap, under the influence of contact- 

 action, the sphene of the andesites and ashes appears to have been 

 re-dissolved and re-deposited, one of the most striking features of 

 the altered rocks being the considerable number of large grains of 

 deep-coloured, very dichroic sphene which have been formed, wholly 

 different from anything seen in the i-ocks at a distance from the 

 contact. Sometimes these large grains of sphene occur in cavities 

 in such relationship to quartz and other infiltrated matter as to leave 

 no I'oom for doubt as to the manner of their deposition. 



Notwithstanding this considerable amount of titanic acid, how- 

 ever, the slates resulting from the chemical and mechanical altera- 

 tion of the andesitic ashes do not ever show the rutile-needles so 

 universally characteristic of the sedimentary clays and slates. From 

 a considerable observation of these rocks from all over the district 

 I am able to state that rutile in that form is never noticed in them, 

 except in those cases in which it is explained by included sedimen- 

 tary matter originally containing it. To the best of my belief, 

 based on much study of this special point, it may be pretty safely 

 said that rutile in the form we know as " slate-needles," — a form so 

 very characteristic and always recognizable, — occurs only as the 

 result of decomposition, under certain conditions, of deposits partially 

 consisting of biotite. As I have noted in a former paper (Gteol. 



' Even now that the general presence of titanic acid in rocks is more fully known 

 than formerly, it is very rarely determined by analysts. This arises to some extent 

 from the fact that its non-determination does not affect the total addition of the 

 analysis, as in the ordinary course of the estimations it is weighed partly with the 

 silica and partly with some of the bases. But the non-determination is more largely 

 due to the fuct that the exact quantitative serparation of titanic acid is a very 

 tedious and dithcult operation. 



In the "American Journal of Science" for December, 1891, is an interesting 

 paper by Mr. F. P. Dunnington, " On the Distribution of Titanic Oxide upon tlie 

 (Surface of the Earth," in which he gives the results of 72 determinations on soils 

 from various parts of the world, as well as on a few rocks. The universal diffusion 

 of this oxide in appreciable quantity is fully demonstrated. These many determin- 

 ations were not made by Mr. Dunnington in the ordinary tedious gravimetric 

 maimer, but by a rapid caloiimetric process which is described in the paper. It is 

 to be supposed that this method has been fully checked and proved to be reliable. 

 That being so, the determination of titanic acid in our rocks and minerals ought 

 to become the rule instead of the exception, since an easy and rapid method is 

 available for it, . . . 



