'Prof. Hull — On the Structure of Trap-rocks. 155 



numerous, such as "Greenstone," '^Basalt," "Porphyry," "Amyg- 

 daloid," "Spilites," "Diorite," and "Melaphyre." Let us hope that 

 the term " Greenstone " will in course of time — and chiefly through 

 the aid of the microscope — fall into oblivion ; at present it can 

 only be regarded as a word intended to conceal our ignorance. 



Assuming the words "greenstone" and " diorite " to be synon- 

 ymous, and intended to designate a crystalline rock composed of 

 hornblende and felspar, it is questionable whether such a rock is 

 actually to be found amongst the Limerick trap series, although 

 crystals of hornblende do sometimes occur as accessories. 



As stated by the officers of the Geological Survey," These rocks are 

 of dark and light green, dark and light blue, or reddish colours, the 

 latter being often due to decomposition. Their texture is generally 

 compact, or fine-grained, with crystals of felspar widely dispersed 

 through the mass ; other crystals are rare, and some even of those 

 traps, which from their dark colour have been called ' greenstone,' 

 seem almost entirely felspathie. Some of these rocks pass imper- 

 ceptibly into porphyries and amygdaloids, which have many varieties 

 in the different localities in which they are met with." Mr. Jukes 

 adds in a note, " A detailed chemical investigation into the compo- 

 sition of these varying trap -rocks would be very interesting and 

 instructive. Without it, it is impossible to guess the proportions in 

 which the felspathie are mingled with the augitic or hornblendic 

 minerals; or even to decide on the nature of the felspars them- 

 selves." ^ 



It is exceedingly interesting to observe that similar rocks occur in 

 great force amongst the Lower Carboniferous beds of Scotland, 

 forming the ranges of hills which bound the Coal-field of the Clyde 

 basin along both its northern and southern borders. They have 

 apparently been erupted from volcanic vents at the same, or nearly 

 the same, period of geologic time as the Limerick traps themselves, 

 while in the "toad-stones," or basaltic amygdaloids of Derbyshire, 

 we recognize the English representatives. Thus it appears that in 

 the regions now occupied by the S. W. of Ireland, the centre of Scot- 

 land, and the centre of England, volcanic craters, in all probability 

 submarine, were in active operation, pouring forth over the bed of a 

 highly populous sea sheets of lava, and vomiting forth ashes and 

 agglomerates amongst forests of crinoids and tribes of molluscs 

 and corallines. When, in 1868, I brought over some specimens of 

 these rocks from Scotland, and showed them to Mr, Jukes, he at once 

 recognized their similarity to "the Limerick trap-rocks." 



The case here mentioned is an instance of the homogeneous 

 character of igneous rocks of the same age over considerable 

 tracts of country, which seems to me to have important bearings on 

 the question of the internal fluidity of the earth below the solid 

 crust. 



Microscopic slices. — Of the twelve sliced specimens which I have 

 examined, taken from various places over the district, all but one 

 belong to that general group of augitic rocks which may be con- 

 1 Explanation to Sheet 44, p. 9. 



