574 GREAT SERPENTINE-BELT OP NEW SOUTH WALES, V., 



they may differ in macroscopic appearance, (they are purple, 

 speckled grey, or green) prove to be porphyrites, composed of 

 phenocrysts of augite and plagioclase, in a very fine-grained 

 base. The exceptional rocks are certain rhyolitic keratophyres. 

 The matrix is a brecciated crystal-tuff, of the same composition 

 as the inclusions, and consists of minute fragments of these rocks, 

 or of their phenocrysts. These are sometimes so closely and 

 regularly packed together that it is difficult, at the first glance 

 through the microscope, to distinguish between the matrix and 

 the inclusion. These features must owe their origin to one or 

 both of two causes : either the mass is a deposit of volcanic 

 ejectamenta, which were rounded by attrition in the vent; or it 

 is detritus from a volcanic cone, which reached above the surface 

 of the water, and gave an opportunity for the shaping of the 

 blocks by wave-erosion. It is difficult to decide which was the 

 paramount factor. In either case, the rapid variation in the 

 size of the boulders, shows that the rocks exposed were deposited 

 near the source from which they were distributed, whether this 

 be the centre of eruption, or the outlet of a valley which cut 

 into the volcanic mass. The very small thickness of the mass, 

 and the very regularly banded and minutely granular character 

 of the sediments above and below the igneous material render 

 the wave-erosion hypothesis difficult of application unless it be 

 considered that this exposure is on the outer fringe of a large 

 mass of ejectamenta, of which there is no other sign in the 

 vicinity. 



Another feature, which we may associate with the explosive 

 action of the igneous eruptions, is to be found in the nature of 

 the limestones. These show many features identical with those 

 described by Messrs. Gardiner and Reynolds from the Ordovician 

 rocks of the Tourmakeady District, County Mayo, Ireland. 

 Besides the normal, massive, coral-bearing limestones, there are 

 to be found, in the Parish of Nemingha, "limestones brecciated 

 VII situ, pink or white rocks, which, after being cracked into 

 numberless pieces, have been recemented by the deposition of 

 material into the cracks," and, even more frequently, ^'limestone- 

 breccias, a coarse type of which contains angular blocks of lime- 



