CoLK — Contact-Vhcnomcna at Junction of Lian and Dolerite. 59 



On scratching -with a knife, it remains black and lustrous, and is 

 probably an iron-ore, the occurrence of which is determined by 

 some original difference of composition in this particular layer of the 

 stratified series. A similar opaque mineral occurs in plates and 

 granules in Portlock's specimens. 



I am unable to determine the minute colourless constituents which 

 fill in the spaces between the prisms and granules of yellow-brown 

 pyroxene. Some are prismatic, like small felspars; others are 

 merely granular. This transparent ground is not affected by hot 

 liydrochloric acid, since the irregular edges of broken fragments of the 

 rock retain their forms, even when examined during the attack by an 

 objective magnifying five hundred diameters. I cannot, therefore, 

 verify the presence of woUastonite, which might very reasonably be 

 expected to occur. 



It is, moreover, a coloured pyroxene that has invaded the shell- 

 fragments of the Liassic sediment, not only in the way of an infilling, 

 but also as a replacement of the shells. Oldham^ long ago noticed a 

 belemnite at Portrush, the cavity of which was occupied by augite ; 

 but he regarded this as resulting from an intrusion of the underlying 

 dolerite. We may, at any rate, agree that such mineralisation-is due 

 to the direct influence of the dolerite, and perhaps to the inflow of 

 crystallisers " from it. Aggregates of pale green-brown granular 

 pyroxene, appearing as long bands when the microscopic section is cut 

 transversely to the bedding of the rock, represent in many layers the 

 substance of the well-known fossils of Portrush. 



This type of alteration, in the fossils and in the ground, is still 

 conspicuous in a specimen of the flinty Lias, taken from the top-bed 

 cf a quarry, where the nearest visible igneous rock lay a metre lower 

 down. Close to the dolerite, the fossils become practically lost. 

 There the micaceous zones also appear. Thus, in the banded specimen 

 particularly studied, and described above in connexion Avith the 

 " bronzite," the layer nearest the igneous rock consists of abundant 

 pyroxene in a colourless granular ground. The streaky grouping of 

 the pyroxene, when read in the light of less altered specimens of the 

 rock, indicates the former presence of shells. Then follows a zone 

 in which the opaque feathery mineral appears, and the pyroxene 

 granules are smaller. Then a zone in which these small granules are 

 associated with the opaque mineral and brown mica. This passes 

 gradually into the normal pyroxenic flinty rock, which is grey and 



111 Portlock's lleport on Geology of Loiuloinlcriy, &c., p. 150. 



