No. 6.] HARRINGTON — NOTES ON DYKES. 317 



a hundred yards, and they possess a well marked columnar 

 structure. Their general bearing appears to approach east and 

 west, but the main dykes occasionally divide, a branch striking 

 off at an angle of from twenty to forty degrees." Some of them 

 have been traced for many miles, cutting both the limestones and 

 gneisses, and sometimes forming a ridge across the limestone and 

 a hollow in the gneiss. Whenever they are seen to come into 

 contact with the syenite they are interrupted or cut off by it, 

 being therefore more ancient; and "the relations" Sir William 

 states u of the base of the Lower Silurian group along the foot 

 of the hills composed of the syenite are such as to make it evi- 

 dent that the Silurian beds in some places overlie eroded portions 

 of the intrusive rock." All the intrusive rocks of this region 

 are, however, cut by a set of dykes the relations of which to the 

 Silurian series is not known. They were described in Sir William's 

 original report under the name of melaphyre, but were after- 

 wards designated by Hunt as dolerites, though differing con- 

 siderably in characters from the older rocks of that name. 



The writer regrets that he has not had an opportunity of 

 visiting any of the places mentioned above, or even of seeing 

 authentic specimens of any of the dykes, with the exception of a 

 few from Grenville and two or three other localities. These 

 specimens have, however, been sliced and studied microscopically, 

 and a few notes on their microscopic characters may be of interest. 



MICROSCOPIC CHARACTERS. 



I. Grenville, Lot 9, Range IV. (Plate, fig. 1) The ex. 

 amination of a specimen from this locality shows it to consist of 

 plagioclase feldspar, augite, magnetite, viridite,* apatite, and a 

 little mica and iron pyrites. The plagioclase forms a very con- 

 siderable proportion of the rock, and although much of it has 

 undergone alteration and lost its transparency, it still shows in 

 places, with polarised light, the banded appearance common in 

 plagioclastic feldspars. It has evidently crystallised before the 

 augite, as blades of it are frequently seen to penetrate the latter 

 mineral. The augite is pale brown or in places pinkish in colour. 

 Its form has, for the most part, been impressed upon it by the 



* This useful name is applied to a number of green substances 

 which often result from the decomposition of augite, hornblende and 

 olivine, and which cannot always be u individualised." 



