276 C. L. Whittle — Ottrelite-hearing Phase, etc. 



there described its occurrence in narrow prisms in this rock 

 together with its multitude of rutile inclusions seem excep- 

 tional. The stouter prisms have terminal faces of an octa- 

 hedron and range in size from -£$ to ■%%-$ of a millimeter in 

 length. As the stage is revolved the pleochroism seems only 

 the intensification of the inherent color of the mineral : browns 

 become browner and blues, bluer. The presence of rutile 

 inclusions shows the anatase to have formed after that mineral 

 and suggests the probability of its being a paramorphic product 

 of the rutile inclusions. 



Rutile dots and prisms exist in multitudes enclosed by all 

 other minerals of a secondary nature. They are so extremely 

 minute that even in a very thin section they focus in six or 

 more different planes. 



All traces of original clastic material in the rock have dis- 

 appeared: feldspar detritus, if it once occurred, has been con- 

 verted into a mosaic of quartz, sericite, biotite and probably 

 albite, and the detrital quartz has been granulated. The 

 existing feldspar is the characteristic untwinned glassy variety 

 carrying quartz and sericite inclusions so common throughout 

 this horizon, and was formed after the granulation of the rock, 

 since granulation could not have taken place without straining or 

 crushing it. Nearly all the quartz is sprinkled with rutile inclu- 

 sions, but it is noticed the larger areas have less of them and 

 may be cores that have escaped granulation. Their presence, 

 however, in such abundance militates against the probability 

 of any of the quartz being allothegenic. and indicates, rather, 

 its secondary nature. In the same way quartz may enclose 

 plates of micaceous ilmenite but does not enclose sericite. 

 There is evidence of two periods of dynamic action indi- 

 cated by a faint wavy extinction in the feldspar, in some 

 instances and by the bending and breaking of ottrelite prisms. 



We have then in this rock three titanium-bearing minerals 

 (ilmenite, rutile and anatase) ottrelite, chlorite, feldspar and 

 quartz. What is their genetic order of development? This is 

 a difficult question to answer without more data and is particu- 

 larly difficult in the cases of the ottrelite and rutile. The 

 relative position of the former mineral can be determined 

 easily, but the source of the solution introducing it is not easily 

 discovered. Ottrelite was formed after the rock had undergone 

 metasomatic and dynamic changes that converted its clastic 

 feldspar to its resulting minerals, after its detrital quartz was 

 sugared and the rock had become a stable aggregation of 

 minerals under the conditions of environment then existing. 

 This environment changing, owing to one or more of the many 

 factors affecting the character of a rock mass, ottrelite was 

 introduced probably and seemingly necessarily from some ex- 



