W. M. Hatchings — Ash-slates of the Lake-Didrid. 221 



Mag. 1891, p. 537), biotite appears to be practically absent from 

 the rocks of the Borrowdale series.^ 



The titanic acid in these slates takes other forms. It largely 

 occurs, as in the altered andesites, as disseminated granules and 

 small crystals of secondary sphene. Slates showing this, to the 

 exclusion of any other form of occurrence, are exemplified in the 

 quarries at the top of Kentmere Valley, and on the other side of the 

 same ridge in Troutbeck Valley. 



Rutile occurs, though on the whole sparingly, in the form of blunt 

 crystals and grains lining cavities, the central parts of such cavities 

 being usually filled in with quartz, or calcite, or both. 



Another and more w^idely-spread form of occurrence is as anatase 

 in small double pyramids, with or vsrithout the prism-band, exactly 

 as in the altered Coniston Flags at Shap.^ These anatase crystals 

 were first noticed in a slate from Honister Crag, and some difficulty 

 was at first experienced in accepting the true nature of their origin. 

 I was disposed to regard them as having been introduced in meta- 

 morphosed sedimentary fragments, as in the case of the tuff at Falcon 

 Crag, and to suppose that the crystals had remained distinct after 

 the other material of the fragments had been completely obliterated 

 and absorbed into the rest of the slate. But longer observation of 

 this particular occurrence, and of numerous others subsequently, 

 found, quite dispelled that idea. There is no doubt that the solutions 

 which have permeated the ash-beds, and which have acted under high 

 pressure (as witnessed by the bubbles in the calcite) and probably 

 high temperature also, have dissolved titanic acid and allowed it to 

 re-crystallize (according to differences of conditions we are not able 

 to specify), either as anatase, more sparingly as rutile deposited in 

 cavities, or in combination with silica and lime as sphene. The 

 anatase occurs mainly as clusters in patches of chlorite. V^ith the 

 perfect crystals are often large numbers of grains not showing 

 definite forms, but apparently deposited at the same time. The 

 mineral occurs also in secondary quartz-grains which are abundant 

 in some of these rocks ; in calcite, though rarely, and now and then 

 in felspar-substance. In some cases cavities are seen lined with 

 rutile, and filled in with quartz in which are perfect little crystals 

 of anatase ; so that between the time of deposit of the rutile on the 

 sides of the cavity and the final in-filling with quartz a change of 

 conditions as to nature or temperature of solutions, or both, had 

 taken place, which altered the crystal-forms of the titanic acid being 

 deposited. 



It may be noted that in slates which show very much sphene 

 anatase does not usually occur. Otherwise a large number of the 



^ It is also to be remarked that the small crystals of tourmaline, so usual in clays, 

 shales, and slates, are never seen in these ashes. 



^ Since the above was in print, I have had the opportunity of examining some of 

 these minute anatase crystals by means of a xb- inch oU-immersion objective. This 

 has enabled me to see them very much better than 1 ever did before, and to 

 ascertain that 1 was mistaken in stating that the prism-band is present on some of 

 them. The apparent b;ind disappears when the crystals are seen under the great 

 magnification, together with fine definition, which the use of this objective gives us. 



