312 Ernest W. Skeats : 



Mr. Hewitt's paper bears evidence of most careful petrologioal 

 examination of the rocks, and his examination led him to the 

 conviction that many of the rocks were of an intrusive character. 

 This view he tested in the field. Mr. Lidgey was then mapping 

 the area, and had come to the conclusion that, the diabase was 

 interbedded with metamorphosed Siluriaij (Oxdovician) sedi- 

 naents. Mr. Howitt got Mr. Lidgey to re-examine the evidence in 

 the field, and as a result of this he found evidence of alteration 

 in the Ordovicians. The nature of the alteration of the Ordo- 

 vician relied on by Mr. Howitt was first a bleaching of the sedi- 

 ments by the elimination of organic matter, then molecular re- 

 arrangement, and in extreme cases silicification to '' adinoles " 

 with over 90 per cent, of Silica. 



While the intrusive character of some of the igneous rocks was 

 placed beyond a doubt by Mr. Howitt's petrological work, I think 

 the evidence given of contact alteration by the diabase was of a 

 less convincing character. For instance, he does not discuss the 

 evidence of the maps published with his paper, in which, while 

 cherts are shown between the diabase and Ordivician in some 

 places, in others unaltered Ordovician is shown on the maps in 

 contact with diabase, regarded by Mr. Howitt as intrusive. Fur- 

 ther, the silicification of sediments to cherts, as the result of 

 intrusions of diabase, is a distinctly unusual type of purely 

 contact nietaimorphism, which would seem to merit some discus- 

 sion, especially as among the altered diabases he records silicified 

 rocks. However, the feature noticed by Mr. Howitt south of Mt. 

 Camel that the Ordmncia.n rocks which are silicified become less 

 so away from the contact, is certainly significant. 



Mr. Hewitt's view that the intrusion of the igneous rocks 

 occurred probably in Devonian times appears, to have been based 

 upon analogy with the age of other intrusive masses, and not 

 upon definite field evidence. Less weight, therefore, attaches 

 to this view tha.n to his conclusions as to the relations of the 

 igneous rocks to the Ordovician series, especia.lly as Lidgey 

 reported the occurrence of metamorphic rocks in the Silurian 

 oonglomerates of Mt. Ida. Professor Gregory's paper, in which 

 he supports Mr. Dunn's view of the pre-Ordo^acian age of the 

 cherts and igneous rocks, is of a very suggestive and far-reach- 

 ing character. 



